Maybe the frogs aren't eating because they feel exposed in the feeding tank or were frightened from being picked up? Maybe they'll get used to it. You could try bowl-feeding, but with three frogs it might be tricky to figure out which one is getting what.

I have had luck with dubias and my frogs seem to digest them pretty well. The only thing is that they're so flat sometimes it's hard for the frogs to get them off the bottom of the bowl.