A good rule of thumb is about 1-3 watts per gallon. They have "mini" heaters but I don't suggest those since it automatically adjusts to 7 degrees above room temperature and doesn't allow you to set the temperature. Also with that is drastic temperature changes can stress your fish!
If you have a 3 gallon, Marineland stealth heaters in my opinion are the best. They are fully submersible and you can adjust the temperture to what is best for your fish (i.e 76 or whatever). Get the 10 watt and put it on the otherside of the tank away from the filter to allow good circulation. Hope that helps :)
P.S tip for heaters: when adjusting to a higher temp such as 76 F and for example your temperature (before adding the heater) is 70 F, slowly increase the temperature. Set it to 72, wait a few hours, then 74 and wait another few hours, then 76.
Last edited by surfnchk; 09-30-2010 at 12:53 AM.