I have a portable 220v heater. I usually put it close to where I'm working at first. As the garage warms up I move it away. Being portable I get instant heat where I'm working. I have one like this.
This is the same one I use to heat my 1 car un-insulated garage. I use the same 240v outlet I use for my welder, but I did have to change the plug on it. It works really well and is portable and convenient. I am sure it would be fine for a 3 car garage if it was insulated. It's basically the equivalent of 4 or 5 15 amp space heaters. If it is really cold out there, I'll run a 15 amp space heater with it just to get things up to speed.
