"We weren't playing as sharp as we normally can," Washington coach Barry Trotz said. "We weren't executing the way we normally did in the last couple of games. We were just off a little bit, and you can't be off in this game to have success. We will be better next game."
The Capitals put part of the blame for the loss on their lack of discipline. They were shorthanded five times, and the Lightning got power-play goals from Stamkos in the first period and Kucherov in the second.
Washington was 0-for-3 on the power play.
The Lightning scored two 5-on-5 goals after scoring one in the first two games.
"We were chasing the game all along," Trotz said.
Stamkos gave the Lightning a 1-0 lead with his third power-play goal in as many games at 13:53 of the first period. His one-timer from the left face-off circle beat Holtby high into the right corner of the net.
Stamkos has five goals and four assists in a six-game point streak.
"It doesn't matter who scores, as long as we win," Stamkos said. "But obviously, it's nice to perform at this time of the year. You always want to step up."
Kucherov made it 2-0 with a one-timer to the short side from the right circle at 1:50 of the second period. He has two goals, each on the power play, and four assists in a five-game point streak.