Not sure about the fruit flies, but it seems they would be super small.
Soldier flies -Larva - Those are the same as Phoenix worms. I feed them to Rin along with crickets or dubai and other worms (same feedings)
When I feed them to him, I toss the worms and dirt they are in, into a bowl, the dirt falls to the bottom and the worms float. I then scoop out the worms from the water, and put them either on the lid of the container or a very shallow bowl, and let him eat them that way. They are cleaned off, and can eat several at one setting, as well as get some fluids into him while eating them. They are very high in calcium as well.
Dubia- For a 3 month old, x-small to small is good. Once he reaches 5 months, small to medium is probably better. Remember the smaller the dubia the less meat is on them. Rin LOVES dubia. I just fed him 25 mediums today, in place of crickets. He KNOWS when I have a bag of dubia vs a bag of crickets. and his eyes get big and he gets so excited when he sees the bag in front of him. (be careful waving a bag of food in front of him, last week, I waved a bag of dubia in front of him, and before I knew it, he had bitten the bag, but thankfully had not bitten it through to the plastic getting into his mouth, he was just so excited!! And I was not paying close enough attention to it being too close to his face.)-lesson learned.
I also believe that variety is best for both nutrition and keeping their interest. Not any one feeder will give him all the nutrients he needs, and in the wild, Im sure they would get more than one type of feeder as well.
I'll let others speak for the rest that you posted about, as I don't feed Rin most of those.
Have you also considered horn worms or butter worms? With a 3 month old, I would cut up the horn worms first, unless you get them very small. But smaller butter worms should be okay with their sizes.
One last thing, I don't know if you feed him inside or outside of his viv, but if you feed him inside the viv, you might want to consider getting a tub that is outside the viv for feeders. They can control them easier without distractions outside the viv, and any that are not eaten, you can put back for the next day, and don't have to waste as much.