The win set up a league-deciding meeting with Houston Dynamo, who beat Sporting Kansas City 2-0 to win the Eastern Conference title.
Galaxy's goals came from Landon Donovan and Irishman Keane, while Beckham provided the cross for the other.
For Beckham, the MLS Cup encounter could be his final game for Galaxy.
The former Manchester United and Real Madrid star's lucrative five-year contract expires at the end of this campaign but he said he was focusing on the Cup, not his future.
"I'm a Galaxy player right now. I'm 36 years old and I still enjoy soccer," he said.
"Right now we're focused on the Cup. We don't want to get too carried away. We did that a couple of years ago [when Galaxy lost to Real in the MLS Cup] and paid for it."
The MLS Cup takes place in Los Angeles on 20 November.