A single space heater connected to a thermostat will do wonders, though most of them have basic dials for the temperature so you may not even need to do that.

Personally my one computer and my snake lights keep my room well over 70 degrees, but if I turn off my computer the temperature will drop a lot, so I know exactly what you're talking about. I mean I don't have 3 computers, but my one running at 110 at all times is enough to keep my room toasty (not to mention the Antec 900 moves a lot of air through there at all times).