well most frogs can climb up glass etc so u will need a lid for it, there was someone selling a frog set up with 2 frogs in adel on petlink for about 200 if ur interested in something thats all ready to go dont no if its still availible tho
if you are not looking for high matinence its best to have just a bowl of water a large one or you could get a piece of glass to fit in you tank nice and snug and then slicone it so its water proof and then fill up just that section and a good substrate is river stones
From how many for a three foot tank to how best to set it up would be determined by what sort of frog you want to keep. There are some care sheets available on frog.org.au/community if you want to have a read to help you decide.