I have to ask, is that 3" total or 3" nose to vent? If it is 3" total that is really a small dragon. Size will be a factor in what food items you can offer. If you offer food that is too large the MHD may try to eat it but it is best with all reptiles to offer food of appropriate size.
I have some babies and the one that is 2 months old is about 3 1/2" long so at 3" you are looking at a very small dragon. I feed them 3/8" to 1/2" baby mealworms, 1/4" crickets, very small red wigglers, or 1/2" silk worms.
If, however, you dragon is 3" snout to vent then that is a different story altogether and it could then eat small nightcrawlers, full-grown mealworms, 1/2" crickets, 1" silkworms, you get the idea.
As for the "where do I get them", you can find several places that sell red wigglers for aquarium fish food or you can raise your own in a tub. I raise all of my own mealworms, am starting to breed orange head roaches, and mice for occasional pinks. Of course the pinks only go to full grown dragons. I rarely mess with crickets anymore as they just don't seem to be a favorite and I hate the smell. I order silkworms from www.mulberryfarms.com and get my wigglers from Kazarie worm farm at www.kazarie.com/ and you can also feed nightcrawlers that are raised as fish bait. Be aware however that fishing worm bait is usually raised in manure and poses a risk of introducing parasites thus increasing the need to have your dragon checked regularly for parasites.
You can find an abundance of worm information at www.happydranch.com/.
You will find a more information on my web site as well at www.froggieb.com/MHDshortcare.html.
Hope this helps.
Keep us posted and let me know on the size issue.
Marcia 
