I'm not sure about heaters rated for 3 gallon tanks, but I would just get a 25 Watt, (10 gallon heater, or so they say) and just mess around with it until you find what setting produces the desired temp for a 3 gallon.
Edit- Actually the 25 watt heater is for 5 gallon tanks, so that would be good for you. I use Visi-Therm Stealth heaters, they are incredibly reliable, and get the best ratings (on the cichlid forum product reviews). Plus they are black so they are kind-of "stealthy" (hence the name)