As Su.Ki. stated, all three are possible. But as Beckie said, "harsh on him" is the most frequently used. It also sounds the most natural. Here are some natural examples of how you can use harsh on/to/with: 1) Don't be so harsh on him! 2) Did that sound harsh to you? 3) That professor is very harsh with grading. I hope this helps. ^^

I saw "on" a lot times, as I know "on" is more common, but 'to, with' are available also. but I'm not sure, wait native speaker's answer :)

They are all possible.

Definitely harsh with:)

it can never be on. its either with or to(better use towards) ppl use on' but its not right grammatically