Replace forced air heater with electric radiant floors
800 watts/room is 2,400 watts, drawing 10 amperes total at 240 VAC. Assuming you need to increase it by 50%, as you suggest, the maximum current is still just 15 amps, still within the 20 A circuit capability.

You must have a very mild climate and well insulated house if 800 W (about 2,700 BTU.hr) is sufficient to keep it warm... you're not up by Snoqualmie Pass. ;-)