Mexican international Francisco Fonseca opened the scoring in the 11th minute at the Home Depot Center before Walter Gaitan and Alejandro Villalobos completed the rout with goals late in the second half.
Beckham, whose scheduled first appearance for the Galaxy against Chelsea on Saturday is in doubt because of a swollen left ankle, watched helplessly from a corporate hospitality suite in the company of club president Alexi Lalas.
The England midfielder, signed by the Galaxy from Real Madrid on a five-year deal worth $32.5 million, had his ankle in an ice-pack.
Beckham twisted his ankle playing for England against Estonia in a Euro 2008 qualifying match last month and aggravated it in his final appearance for Real Madrid 11 days later.
He has undergone regular treatment over the last three weeks and is eager to play for the Galaxy at the Home Depot Center on Saturday in what has become a hotly anticipated debut for the major league Soccer side.
Whenever Beckham does don the Galaxy jersey for the first time, he will face an uphill task in trying to inspire his new team mates.
The Galaxy have won only three of their 12 MLS games this season and languish second bottom in the Western standings.
They also failed to reach the U.S. Open Cup quarter-finals after losing 1-0 at lowly Richmond Kickers last week.
(Writing by Mark Lamport-Stokes in Los Angeles)
Image: David Beckham and LA Galaxy CEO Alexi Lalas (left) watch the LA Galaxy vs Tigres UANL match on July 17, 2007. Photograph: ROBYN BECK/AFP/Getty Images