I'm betting her failure to thrive and grow is due to not enough insect protein and dietary calcium , and likely inadequate UV and wrong viv temperatures unless she sharing a viv with another .
For starters @ 3 months old she should still be on three live insect meals per day since she is still a hatchling.
The best feeders for her at 3 months old are :
BSF maggots (phoenix worms (USA) , calci-gents elsewhere) = excellent feeder naturally rich in calcium and slow moving
Blowfly gents (rich in calcium and slow moving BUT pupate fast, but the pupae and the flies a good tucker for a hatchling too
Silkworms (1 inch - 1.5 inch long) excellent feeder insect - slow moving worth the expense and bother raising on either fresh mulberry leaves or silkworm chow
Crickets (I'd be offering 25day old - 1/3 size , no bigger for now), roaches, locusts (suitable sizes) which are feed and gutloaded on carrot + adult beardie pellets (repcal , straight from the bottle) + calcium rich greens and these need to dusted lightly with calcium powder.
As many bugs as she will eat (in her viv) offered a few at a time so she's not overwhelmed with the fast or hoppy feeders.
First feed about 2hrs after lights and heat on, last feed about 2hrs before lights /heat off. At least a 14-15hr photoperiod to give lots of opportunity to bask and warm and digest food.
Need much more information and details on your beardie's setup and your husbandry otherwise we can't even hazard a guess.
Here's my Cheat Sheet :
viewtopic.php?f=6&t=234738&p=1806050#p1806050
if you work through it methodically , the chances are the likely issues that need to be tweeked or changed will pop right out at you, if you can give very detailed answers we will then be in a position to offer help.
Is impossible to help you if you don't provide thorough details .
It may prove necessary to involve a reptile vet if your bearded dragon has serious health issues.
Are you in a position to afford to take him to a good reptile vet should this prove necessary ?