Why would you not put a heater in a 3ft tank? I understand the problem with small tank/jars, but if you're going to have a 3ft tank you may as well heat it (don't risk your fishes health to save $15). But if the temperature of the room is constant and above 75 degrees, then I guess you wouldn't need one, BUT if it's not constant and not above 75, get the heater.
Linda