Rules and other rules

There is a difference between "rules of the game" vs "rules about playing the game". "Rules of the game" are purely logical ones - all that is needed to play (or referee) by a computer. i.e. The board and piece powers, the actual moves, prohibitions and priorities. "Rules about playing the game" are specifically for humans; they are physical rather than logical. i.e. playing time, what to do about irregularities or illegal moves, whether things like "check" have to be said out loud, fast scoring methods, blowing smoke in your opponent's face, etc.

In three player games, it seems a good rule to say "it is illegal to leave a next-player immediate win, if preventable". Also, if player A wants to make sure the next opponent plays to block the 3rd opponent from an immediate win, he must say, "B, C is about to win, please stop him which you can do by playing this". Then B is physically obliged to stop C, and A gets the proper reward for his forethought. But if both A and B overlook that C has a win coming up, A will say nothing, B will fail to prevent it, and C will duly win, without (a legally required) takeback, and profit from HIS own alertness. This is a good compromise that does not affect the purity of the rules of the game and makes it the responsibility of the previous previous player to warn that danger is at hand. This is fair since it's the previous previous player who benefits from all this anyway.