If you would like to keep a Betta with your 3 frogs, I would get a 10 gallons, you never know, your betta can be nippy or as frogs do not have good eyesight they might nipped at the betta's tail thinking it's food, I would either get a 10 gallons for everybody or keep the 5.5 for the frogs and the 3 gallons for the Betta