You can't plug a 3kW heater into a standard 110/115/120V 15A/20A circuit.
That's why you should use 240V circuits.
I just rewire my 120V outlets onto 2-pole breakers and voila. I just be sure to label them and make sure I don't plug anything into them that can't take the voltage.
A bit safer would be to also replace the outlets with 240V-style outlets, and get 240V-style IEC cables:
http://www.ebay.com/itm/6-15P-to-IEC-C13-Power-Cord-3-Foot-15-Amps-250V-/111172349097
There's a couple huge caveats with what you propose if you do want to do it.
1) You should be the homeowner.
2) You should do a new run of wire to dedicated outlets. It's honestly not that tough, and most homes have god-awful and undocumented branch circuits. Better safe than feeding 240V to some random outlet you didn't know was on the same branch.
3) If you do rewire an existing line, absolutely replace the receptacles with NEMA6 ones. Leaving 240V on a NEMA5-15 is a disaster waiting to happen.
Seriously though, just set it up properly. I just replaced my old NEMA6-30 for an electric heater with a NEMA6-30L and bought one of these off eBay.
http://www.ebay.com/itm/171154216652
Not only does it let me run everything from a single 30A 240V breaker, but it can be easily wired safely and to code, it meters the current and so functions as a basic 240V KillAWatt and I get 10 proper 240V outputs that are individually switched and I can power cycle remotely through my network. For $50 plus the $30 for the new 6-30L, it's a pretty simple and cheap solution that's a lot safer than repurposing existing branch circuits to be 240V.
