your wall heaters sound like they're on the same circuit breaker as the outlet you're plugging the heater into. Most circuit breakers in home4s are either 15 or 20 amps. Usually 15 amps if you have 14 gauge (thickness) wire, or 20A is you have 12G wier. a 15A circuit breaker at 120 volts can supply about 1800 watts of power. Since your heater is 1500 by itself, that doesn't leave very much for anything else, definitely not enough for two more heaters. In my opinion, your simplest fix is to buy a 12 gauge extension cord (MUST BE 12 GAUGE, or thicker, like 10 gauge, but you probably won't find a 10g extension cord). Plug the extension cord into an outlet in another room, a room without wall heaters. Depending what else is on that other circuit though, you still may trip a breaker. The main thing to keep in mind is converting the watt rating of heaters, to the amp rating of circuit breakers. It's P=IV, Power = current X voltage. So if you have a 120V system (e.g. you live in the US), and your circuit breaker is 15A, then the most you can plug into that circuit is about 1800 watts, TOTAL... That is, counting everything plugged in that's on that breaker, and sometimes they may even run lighting on the same breakers as outlets. If it's a 20A breaker, then you can run 2400 watts total. If you do by an extension cord to plug it in at a different part of the house, like i said, make sure it's at least 12 gauge (smaller numbers are LARGER, so don't buy a 14g extension cord)! It's still not recommended, but make sure not to use anything smaller than 12g. It may be expensive (for an extension cord), like $50 for 50' cord, $30 for a 20' cord, etc...