camelback mini M.U.L.E. is a good one. have a pink one for my daughter, although she's 8, not 3. but i got it for her a couple yrs ago (i think she was 5 at the time) and it has been a good size for her for several yrs now.
i think it only has one molle strap on the back, but it's a good pack.
also High Sierra has some inexpensive biking/hydration packs that work well for kids. no molle straps that i remember, but good, inexpensive packs. they have them at ShopKo up here in WA state, don't know if you have ShopKo's where you are.
-----quick add on. i tried the straight hydration packs - the mostly hydration reservoir, with one tiny little zipper for keys/wallet/etc. they look perfect, but don't bother. they are the right size for a kid fit-wise, small for a small torso. but they hold nothing other than the water reservoir. they can't hold a snack, can't hold any special sticks/rocks/stuff they find, can't hold a long sleeve shirt in case they get cold, can't hold their barbie/stuffed animal they want to bring then get tired of holding/etc. voice of experience, skip those ones.