Well.. 3-5 is pretty shallow for an ACF. 10-12 inches would be better, ACF have no real difficulty with water depth since they are relatively strong swimmers. Also you'll want to utilize as much water real estate in your aquarium as possible, since ACF are messy creatures, the more water the better.

The absolute minimum is a 10 gallon though I do not recommend it. A 20g long is ideal for one frog. I'm not sure what an 'airtank' is but do leave space from the surface of the water to the lid to allow your frog to breathe air and you will need a tight fitting lid, since these frogs are quite adept at escaping an aquarium without one (and they will find ways out of even small gaps in lids, trust me).

Floating plants like indian fern (water sprite) or pennywort or similar floating plants are a nice addition for these frogs since they enjoy resting in it, I also hear it reduces their instinct to attempt escape too. They also soak up extra nitrates in your aquarium which will improve water quality and reduce algae, especially the fast growing plants.