Well, just to turn HF on/off just two positions are needed. I'm not sure it makes sense to complicate the flight mode controls just for that. And having that on/off on a separate control is much more flexible while flying.
The main use for the 3rd HF control position is to set the reference heading, either on the ground or while flying. The secondary use for the 3rd position is to enable "dynamic HF" mode which continuously sets the reference heading based on home position (so back stick is always back to home). The latter two functions could be removed from a switch/control altogether (so you can only set heading while disarmed and there's no dynamic mode which is a bit of a gimmick anyway ).
-Max