Per the Golden Knights' official Twitter account, Shea Theodore, Nicholas Roy and Nolan Patrick were added to the NHL's COVID protocol list shortly before 8:00pm ET, just two hours before puck drop.
At this point, since we have not yet heard otherwise, it appears as though the game will go on as scheduled, with Vegas calling up a trio of players (Jake Leschyshyn, Jonas Rondbjerg and Daniil Miromanov) from their AHL affiliates, the Henderson Silver Knights on short notice.
The Golden Knights did reveal any confirmed positive test results, so the absences of Theodore, Roy and Patrick could be precautionary, but the Leafs have already dealt with a significant outbreak this season, just before Christmas. I don't think there is an appetite for another.