Well assuming black tries to oppose mate, I only see one white move to force mate in 3 moves. Black has three different defences and to one of those white has two possible follow-ups.
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1.Rg5+ Kh6 2.Qh4+ and mate next
1...Kh8 2.Qh4+ or Qh1+ and mate next
1...Qg6 2.Qxg6+ and mate next