This variant is quite simple, but the game play it introduces is sophisticated and games can lead to very many tactical opportunities.
The winning move has to be legal, e.g. you can not give the third check while in check. Checkmating your opponent is still a legal way to win.
A move is considered to give ONE check if the King is threatened, regardless if multiple pieces are giving the check (in the event of a double check).
You can join the Lichess Three-Check team for more content and community-driven events!