I am using TrackIR and normally i zoom with it too.
The older ArmA games used analog zoom: you could zoom in and out smoothly.
in ArmA3 if you zoom in or out the zoom just snaps at some point to fully zoomed in or out.
It is not even connected to TrackIR, you can feel the exact same thing with key bindings. You should be able to adjust your zoom, not just "full or nothing".