Have your main target any mob, then switch to your third toon's window and press Shift-F3.
If that works, then the problem is in your ISBoxer settings.
No that doesn't work either.

I went through my key bindings and I do not see SHIFT-F3 or SHIFT-F4 bound to anything at all except for the spot where I put my macro. Which is on Right Action Bar Slot 7 and Slot 8 respectively.

I can't work with it now, but I'm going to try a suggestion that Lax made about using ALT since the game doesn't use that, so it would be ALT-F3 and ALT-F4.

I'm also going to try a different action bar and see if that works.