Your going to need to get either 3 Ranco thermostats at 75 smackers each. ORRRRRR you can invest in a Herpstat Pro, which easily controls 4 different temperatures. But they're a little pricey, 300+ bucks. i have one though, and its a godsend.

You COULD use dimmers on your heaters, I don't like it, so I don't recommend it, but that is an option.

HTH