It would be great if you could post pictures of your progress! Green frogs can get pretty big and they can definitely jump like crazy. A 40 breeder is WAY too small for even one in my opinion. For three I would go for a 100 gallon so they are more comfortable. Another thing to consider is making sure there's a little height to the tank as well to prevent escape attempts.

I don't know a ton about green frogs, but they are native to Maine and whenever I see them they are either in the water or right on the water's edge. So I would do more like 2/3 water 1/3 land. You could collect native water plants to go in the water and some semi aquatic plants for the land like "jack in the pulpit" (not sure of the scientific name). I highly recommend checking out Bill's build thread for his paludarium: http://www.frogforum.net/vivarium-te...aludarium.html. This is a great model of what you could do! Good luck and enjoy