What is the longest forced win with 3 to 6 pieces (ignoring 50 & 75 move rules)? I am wondering, what endgame, with 3 to 6 pieces, has the longest checkmate with optimal play? Right now, the longest I'm aware of is the two knights versus one pawn endgame, which in certain positions can take up to 115 moves for the knights to serve checkmate.
The 50 move-rule &amp; 75 move-rule are to be ignored.
Longest known checkmate is in 549 moves, revealed by Lomonosov Tablebases.

You can see the game here.

Plus, you don't need to say "endgame" explicitly. Longest checkmates come from endgame tablebases already. And it's not possible to build a middlegame tablebase practically, because of the exponential space complexity. Let's say there is a middlegame position which is in 12-men tablebase:

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5-Men  |  ~7GB
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6-Men  |  ~1200GB
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7-Men  |  ~140000GB    
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8-Men  |  ~10000000GB Estimated
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12-Men |   Do the math :-)
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