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  1. Rxa6 Qb8
  2. Bd5 Qb4+
  3. c3 Qb8
    It is true that white wins, but that's not the fastest checkmate ...
    In the start position white can checkmate within five moves!

By the way: you 'forgot' the upvote! :)

Ah! I always forget to upvote D:
I'll try

  1. Na2 Qb8
  2. o-o-o QxNa2
  3. Rh1#

That's it (also other black moves don't help)! You are the winner of this round! :)

Thank you! :) I have finally solved one of your difficult puzzles after so long. Nice job @jaki01