It was the first career Stanley Cup Playoff hat trick for Marner, who also set an NHL postseason high with four points.
Vegas leads the best-of-7 series 2-1. Game 4 will be at Honda Center on Sunday (9:30 p.m. ET; ESPN, SN360, SN, TVAS).
Shea Theodore and Brett Howden each had a goal and an assist, and Carter Hart made 31 saves for the Golden Knights, the No. 1 seed from the Pacific Division.
Vegas forward Mark Stone appeared to sustain a lower-body injury late in the first period. He briefly returned to the bench early in the second before heading back to the locker room for the remainder of the game.
Beckett Sennecke and Chris Kreider scored for the Ducks, the No. 3 seed from the Pacific. Lukas Dostal allowed three goals on eight shots for Anaheim before he was replaced by Ville Husso at the start of the second period. Husso finished with 17 saves in his second relief appearance of the postseason.
Theodore gave Vegas a 1-0 lead at 1:06 of the first period. Jack Eichel passed the puck from the corner to Theodore trailing on the play and he scored with a wrist shot from the right hash marks.
Brayden McNabb’s short-handed goal extended it to 2-0 at 12:13. The Ducks were on their second power play when Marner picked off a pass in the Anaheim zone and sped the other way. Marner pulled up above the left circle and dropped a pass for McNabb, who skated into the same circle before scoring with a snap shot over Dostal's left pad and under his glove.
Marner’s power-play goal pushed it to 3-0 with five seconds left in the first when he scored on his own rebound from the side of the crease.
Marner’s second goal made it 4-0 at 9:19 of the second period. He received a pass as he skated through the right circle and waited out Husso before tucking the puck into the net from in close.
Marner completed the natural hat trick at 17:56 to extend it to 5-0, bringing the puck out from behind the Anaheim net and scoring short side from the bottom of the left circle.
Sennecke made it 5-1 at 6:30 of the third period when he beat Hart to a rebound in the crease by diving in and pushing the puck into the net off a rebound.
Kreider cut it to 5-2 at 15:09 after scoring off a feed from below the goal line by Troy Terry.
Howden scored into an empty net with 1:56 left for the 6-2 final.
