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. ^^
18 กรกฎาคม 2015
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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 :)
11 กรกฎาคม 2015
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They are all possible.
12 กรกฎาคม 2015
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Definitely harsh with:)
11 กรกฎาคม 2015
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it can never be on. its either with or to(better use towards) ppl use on' but its not right grammatically