Nice to see someone else take an interest in the dwarfs.The 3 frogs you plan to get will be very comfortable in the 5 gallon.Make sure you have a filter that does not cause a whole lot of current.The frogs like their water to be still as possible.Also make sure you have a heater and therometer.I would also highly recommend that you take the time to learn about cycling your tank and doing it before you put your frogs into it.That will be one thing that will really get you off to a good start.
Also try to offer a fresh,varried diet.No freeze dried foods.Just frozen and live.
Check out this site.It is valueable and has a welth of info as well as some good links and a message board.
www.pipidae.net