The fan doesn't know if it is a 2-speed or a 3-speed. Only difference is the switch and the resistor(s) supplying (or limiting the supply of) power to the fan motor. More speeds won't make high speed any higher.
I have never seen a cable/fan speed switch combo with more than the 2-speeds.
If you want to go to a separate fan speed control knob, the options are limitless. You can even do a variable speed fan with near limitless range of speeds between nothing and full speed. But full speed (high fan) is still full speed, no faster. At least on the control side of things. There is the old blower motor upgrade. Not so much with the fan, it has to fit the housing. And there are still limitations to how much air can be forced through the ducts.
What most people have found to get the heater working better, you don't need more airflow. You use a better thermostat. The difference between a 180 and a 195 thermostat doesn't sound like much. But it is a world of difference in heater output.
