I thought he meant 'Green frog' as in Rana Clamitans. (Pretty much a smaller version of the bullfrog) If that is the case I would say a bigger tank would be needed for three green frogs. They tend to be skittish from what I have heard and can jump pretty far, so I wouldn't want them to get scared and jump into the glass. I am not an expert though so hopefully someone else will know more.