Nick Suzuki, Brendan Gallagher and Mike Hoffman also scored for Montreal, Mike Matheson had two assists and Sam Montembeault made 24 saves.
“There were a lot of good things actually on the ice,” Armia said. “I think we played a really good game through every period. I’m just really happy to be back.”
Dylan Strome and Nicklas Backstom scored for Washington and Darcy Kuemper made 25 saves. Washington has lost three straight.
Freshly eliminated from playoff contention, the Capitals played a man short with forward Anthony Mantha ruled out with a lower-body injury.
“When you’re on the ice, I don’t think you’re thinking about if you’re in the playoffs or not,” Strome said. “The hockey sense takes over and I think it was just some not great reads, not great balances and all of a sudden they get two short-handed goals and they had some nice shots.”
Montreal took the lead with a second short-handed goal at 7:52 of the second when Armia whipped in a wrister from the left faceoff circle.
Drouin then located Gallagher in the slot with a backhand pass behind the net to get the Habs a 3-1 advantage.
Armia scored his second of the night, sending a slap shot past Kuemper, giving Montreal a 4-1 lead by the second intermission.
Armia completed the hat trick into an empty net with 17 seconds remaining.
Capitals: Host Florida on Saturday night.
Canadiens: At Toronto on Saturday night.
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