I have the 3 knob control unit too. It's all you need.
It goes like this: Turn the knob on the left to heat (which is the little icon on the right of that knob). Turn your temp to about right in the middle (the blue/red temp control knob in the middle). Do NOT turn the fan knob on at all. Push the center of the knob on the left and the heat icon will start flashing slowly.
Next, wait by the heater vent inside. You should feel some air start to blow as it starts up. After a couple minutes you'll hear the burner start up inside the heater, kind of a rumble. Give it about 3 or so minutes and the fan speed will increase on it's own and start putting out heat. It cycles on and off as it needs to. You never touch the fan control knob (right side knob) on the Tripac 3 knob controller. That is just for fan speed on the AC side.
Sometimes mine doesn't fire the first time and I have to turn it off and start again. If yours won't fire at all the fan will eventually turn itself off and throw you an error message (A little triangle with an ! in the center lights up). Mine does this sometimes because it needs to be cleaned.
There is lots of info on this on here and online. Youtube videos of how to take it apart and clean it, or ThermoKing can overcharge you to do it. If yours smokes out of the outside exhaust pipe and never starts up like I described above then you need to clean it or have someone clean it.