Ive been using locusts as my beardies staple diet since I got them at 4 weeks old. Make sure you get small locusts for 3 week old dragons though. I made the mistake feeding mine meduim to pretty damn large at the beginging for 2 weeks. There perfectly fine though but you run the risk of them not being able to digest it but there little troopers and in the wild im pretty sure they'd go for even bigger ones if they were around.
I just noticed you said 3 months old not 3 weeks How big are they now? Also rankins are the smaller bd's arn't they? Yeah ask your shop if they can get in smaller locusts etc. Or what i did was order from livefoods a half bulk bag of locust and you can choose your size, I got small/meduim and they were perfect for my bd's. I got around 150 locusts in the bag and only about 1 or two out of the lot were dead when they arrived.