I totally agree about adding healthy, live staple insects to their diets at least 2-3 times a week, this is not optional at all. They cannot exist solely on greens and veggies, even if they are only appropriate ones like Collard Greens, Mustard Greens, Turnip Greens, Escarole, Endive, Dandelion Greens, Swiss Chard, Bok Choy and Pak Choy, and Arugula (No Kale and No Spinach at all if you can help it, they both contain very high Oxalate content, which binds to all available free Calcium they take in, and this keeps them from absorbing any of the Calcium they take in, and instead they just "urinate" or excrete the Calcium Oxalate out; this is what often causes kidney stones in humans and in reptiles both). So I'd be willing to say that the fact that you are not feeding them any live insects at all is up to 50% of your problem and the reason that they are so skinny. You need to pick a healthy, live, staple insect (never feed a dragon any freeze-dried or already dead insects, they contain no moisture, little nutritional value at all because it is all depleted from them, and they often cause bad impactions, their insects must be live and appropriate for dragons to digest) and for 3 adult dragons I would be ordering your live staple insects from an online vendor in bulk, as this will be cheapest for you by far. I had my first dragon from a month old up until he died at just shy of 13 years old, and he ate live insects pretty much every day of his life, as already mentioned above. He got a large salad every day that consisted of only the greens listed above along with lots of FRESH veggies such as multiple types of squash, bell peppers, green beans, peas and pea pods, sweet potatoes/yams, some carrots, some broccoli, etc. He also got a small piece of fruit once or twice a week, but that's it, as most fruit is full of sugar that they don't need.
You need to start buying either live Crickets, Dubia Roaches or one of the other species of roaches, Silkworms (hard to find live online), or BSFL/Phoenix Worms/NutriGrubs/CalciWorms (all the same thing, just different brands). I currently have 3 dragons, one male that is a year and 3 months old, a female that is just turning a year old, and another male that is 6 months old, and I feed BSFL worms as my live staple insect, and I supplement them with 1-2 Superworms a day at most (no more than that, Superworms are fatty). Obviously the 6 month old eats pretty much ALL live insects and not many Superworms, he is getting a mix of BSFL and Dubia Roaches until he hits a year old. But the bottom line is that you cannot expect a Bearded Dragon to exist only on Greens and Veggies and stay healthy and of a good weight. They must have live insect protein at least 2-3 times a week, and if you choose to only feed them the live insects 2-3 times a week instead of every day, then on those 2-3 days you must give them a 10-15 minute live insect "Feeding Session" where they are allowed to eat as many of the live insects as they want to, and then you give them their fresh Greens and Veggies AFTER they eat their live insects. This will ensure that they are getting enough healthy protein to build muscle, as now they are getting little to no protein at all.
The other half of your issue is totally inadequate
UVB light, for a couple of reasons. First of all, no UVB tube at all, regardless of what brand it is or whether it's a very strong T5 tube can be at a distance of 20" away from their basking spot. That's much too far away from their basking spot/platform, and they have not been getting much adequate
UVB light at all. Even a 12% Arcadia T5 UVB tube, which is about the best
UVB light you can buy for a Bearded Dragon, must be within at least 11-12" of their Basking Spot. At a distance of 20" away from their Basking Spots, regardless of whether it's an adequate light or not it's not going to be able to get any adequate
UVb light to them while they bask. It's good that you have them mounted underneath the mesh lid (which blocks up to 40% of all
UVb light), that's a must, especially at 20" away, but you need to figure out how to lower the UVB tubes over their Basking Spot/Platforms or raise their Basking Spot/Platforms up to within at least 11-12" if they are T5 UVb tubes, and within 6: if they are T8 UVb tubes. Also, I hope that all of your UVB tube fixtures have Metal Reflectors in them sitting behind the UVb tubes, so that the
UVB light can be reflected throughout their large tanks, if not you need to order some Slip-On Metal Reflectors (Amazon sells them very cheaply, Arcadia brand) that are the length of eatch UVB tube.
*********As far as the 2 types of UVb lights you are using, the Exo Terra UVB Desert Glo UVb tube, the 40 watt version, is only a T8 strength UVB tube!!! This means that it must be mounted underneath the mesh lid, which I believe it you do have it mounted unobstructed and under the mesh lid, However, a T8 strength UVB tube must be at least within 6" or so of the Basking Spot/Platform!!! If you have it at a distance of 20" away from their basking Spot, they are not getting much
UVB light at all!!! I know people who are using the Desert Glo UVB tubes successfully as long as they are the 10.0 version and they are mounted underneath the mesh and within 6" of the basking spot. So this is definitely a lot of your issue! You really need to figure out how to get that Desert Glo within at least 6" of them, otherwise none of the
UVB light from that tube is reaching them at all. Also, make sure there is a metal reflector inside of the fixture...***
****The other UVb tube you're using, the "Trixie" brand tube, is a totally inadequate
UVB light. Trixie is a German pet supplies company that does not specialize in Reptile products, but rather they sell products for ALL types of pets, dogs, cats, birds, rodents, etc. and they do not even manufacture their own Reptile lights, they import them from China, just like the Pet Smart house-brand "All Living Things" does, as well as "Reptile One", and they are all essentially the exact same lights. Again, this tube is not only again a T8 UVB tube that must be at least within 6" of their basking Spots and at a distance of 20" they are not getting any
UVb light to them at all, but in the case of this UVB tube this might be a good thing, as these are one of the many off-brand lights that emit harmful light rays as a byproduct of the manufacturing process, and are well know to cause eye and neurological damage. So the fact that you have this tube so far away from their basking spot has probably saved them from developing severe eye problems. *****
So you absolutely need to figure out a way to get any T8 UVb tubes within at least 6" of their basking spots, unobstructed, for them to get any adequate
UVb light. The Exo Terra Desert Glo 40 watt T8 UVB tube can be used successfully, but again, not if it's 20" away from their basking spot, because it is only a T8 tube it must be within at least 6" of them. And you need to replace that Trixie brand tube ASAP with either another Exo Terra Desert Glo or a better UVB tube, like either a Reptisun 10.0 or an Arcadia brand.
You have very tall tanks apparently if your UVB tubes, which are apparently strapped to the underside of the mesh lids and inside the tanks, are at a distance of 20 inches away from their basking spots. My best advice to you (especially since your dragons have not been getting any proper
UVB light for quite a while) is to upgrade to T5 strength UVb tubes, which are far more adequate for such tall tanks. While you'll still need to mount them inside the tanks and under the mesh lids, since the tanks are so tall, the T5 strength tubes only need to be within 11-12" of the basking spots. This is going to be much easier for you to accomplish than getting the T8 tubes within 6" of the basking spots. Since you must get rid of that Trixie T8 UVB tube (I DEFINITELY WOULD FEAR PUTTING THIS BRAND OF UVB TUBE WITHIN THE NECESSARY 6" OF THEIR BASKING SPOT, AS THEY ARE LIKELY TO DEVELOP EYE DAMAGE OR NEUROLOGICAL ISSUES BECAUSE OF IT, BUT AT ANY FURTHER AWAY THE T8 TUBES WILL NOT GET ANY
UVB LIGHT TO YOUR DRAGONS, SO THIS IS WHY YOU NEED TO GET RID OF AT LEAST THIS PARTICULAR TUBE). I'd take this opportunity to upgrade at least that particular tube to either an Arcadia 12% T5 UVb tube or a Reptisun 10.0 T5HO UVb tube, and get either of these mounted within at least 6" of their basking spots.
As far as the Exo Terra Desert Glo T8 tube, you need to get it within 6" of the basking spot ASAP.
Since you have to replace T8 UVB tubes once every 6 months anyway (that is the age that all T8 UVb tubes decay and stop emitting any
UVB light at all; this is why you end up saving a lot of money by switching to T5 UVB tubes anyway, they only need replaced once every year), whenever your current Exo Terra Desert Glo T8 turns 6 months old and you have to replace it anyway, I'd also upgrade this one to either the Arcadia or the Reptisun 10.0 T5 UVB tube as well. But right now, since you said it's only 2 months old, so you have 4 months left on the Desert Glo T8 tube, and since it's a decent T8 tube, you can still use it for the remaining 4 months it has left as long as you get it down within the 6" of their basking spot. The Trixie brand UVb tube needs tossed now, it's not doing them any good at all at 20" away from their basking spot, and putting it within 6" of their basking spot will no doubt cause more harm than good. So please upgrade that tube ASAP.
Once you separate the 2 dragons that are being housed together (not optional, they need to be separated ASAP, at least with a divider), and you replace that Trixie T8 UVB tube with an adequate one and make sure it is mounted within either 6" if replaced with a T8 or within 11" if replaced with a T5, and you get the Desert Glo T8 tube from 20" away to 6" away from their basking light or replace it with a proper T5 tube an get it mounted within 11", and you start feeding your dragons live insect protein at least 2-3 times a week (every day is better), they will start eating more as their appetites will pick up , they will start to put weight back on, they will have much more energy, and they will all be happy and healthier. Right now they are not well from a total lack of adequate
UVB light, an improper diet that contains no live insect protein, and the 2 that are housed together are suffering from dominance stress....