Gypsy: < Zaius: ... I am not familiar with over the board manners. What is the purpose of announcing "mate in three"?>
The way I understand it, once there was an unwritten honor code that if a mate-in-n was anounced then the purpose of further play (slash post mortem) was to let the mating mechanism shine in the best possible light. That commonly required play all the way to mate along the longest line, but sometimes to resign right after the star move, or at the right fork where equally delightful patterns appeared in several branches.
That era is gone, of course. Anouncing a mate-in-n is showmanship for sure. It can be a quite scary form of showmanship. One can end up with a good size egg on his or her face if the mate-in-3 turns out to be a mate-in-5 or, perish the thought, a piece-up endgame. In such a case the losing side rightfully gains a moral victory.
And when you ever hear this exchange: "Check, and ... erh ... mate-in-n!" .... "Hmmm ... OK, I guess you will have to show it to me -- Rxf3," you know the bragging stakes are high.