The problem is going to be if the heater/thermostat fails and heats the water constantly.
I'd estimate that it will heat 3 gallons of water maybe 15 degrees over room temperature, you'll know better than anyone what your room temperature will be, and how diligent you are at checking the floating thermometer in the tank that you may have
you should be fine in all honesty, the water may get a little on the slightly too warm side, but check the info that comes with the heater (or online if you didn't buy yet) and see if you can find out how much it will heat 3g of water (or double the number for 6g... etc)
also pay attention in a small tank that you can situate the heater correctly.. some need to be kept upright and submerged for example.. so make sure that there is enough physical space to do so, with a little extra space to take into account evaporation.