Wiktionary
n. (context Canadian football English) A timeout and warning given to the coaches with three minutes to go before the end of each half; after this point, the clock stops at the end of every play. Analogous to the two-minute warning of American football.
Wikipedia
In Canadian football, the three-minute warning is given when three minutes of game time remain on the game clock in the first and second halves of a game. (If the football is in play when the clock reaches 3:00, the three-minute warning is given immediately after the ball is declared dead.) The three-minute warning stops the game clock in all cases.
Usage examples of "three-minute warning".
I did not find one but was beginning to see some shape to the curve when the terminal gave me a three-minute warning I had punched in.
At the three-minute warning, a familiar voice from somewhere behind him startled Skeeter into glancing around.