If Haman's Plot said "rip this card to discard three characters", then Gallows wouldn't really do anything (except provide anti-ignore capability to Persians of course). However, it says "discard three, then you must rip." There is no provision for "reversing" the discard if it is not ripped, you just have to complete as much of the ability as you can. Since Gallows is not optional, you can't complete the entire ability, you complete the first part and instead the second part.
