Randy Wells (2-0) continued Chicago's string of strong starting pitching performances, allowing two runs in seven innings on the heels of wins by Ryan Dempster and Ted Lilly earlier in the series.
He also pitched with a huge lead thanks to homers by Kosuke Fukudome, Tyler Colvin and Geovany Soto off Brewers starter Dave Bush. Chicago scored nine times in the first four innings against Bush (1-1) and had 18 hits.
Everyone in the starting lineup had at least a single and Theriot recorded multiple hits for the fifth straight game.
Theriot finished 5-of-6 with two runs and three RBIs and is 14-of-25 in his last five games. He again got it started for the Cubs with a leadoff single in the first and scored three batters later on Colvin's two-run double.
Alfonso Soriano's sacrifice fly made it 3-0, and Fukudome hit a two-out, two-run homer in the second. Colvin added his solo shot in the third and Soto's homer on the first pitch from Bush in the fourth put the Cubs ahead 7-0.
Fukudome followed in the fourth with a two-out double and Lee hit a deep drive to left-center field well over the Brewers bullpen into the second deck that gave Chicago a 9-0 lead.
Milwaukee finally got a run in the bottom of the fourth on Gregg Zaun's groundout, but Theriot's double in the fifth off reliever Claudio Vargas drove in two more to make it 11-1.
Bush stayed stoic on the mound, but showed his frustrations at the plate. After thinking he'd forced Wells to walk him and instead being called out on strikes in the third, Bush threw his head back and slammed his bat on the ground.
It appeared as if manager Ken Macha wanted to stick with the right-hander to help conserve the bullpen, but Bush couldn't make it out of the fourth and left allowing 10 hits and nine runs. Milwaukee was forced to use five relievers.
The Brewers had outscored Pittsburgh 36-1 in the previous three games, only to watch the Cubs outscore them 25-4 in this series for their first three-game losing streak this season.
The Cubs got dazzling performances from their starters throughout the series. Dempster allowed one run in 7 2/3 innings on Friday night, Lilly threw six shutout innings in his first start after shoulder surgery in the offseason on Saturday night and Wells was nearly as sharp.
He stayed out of trouble most of the day and limited the Brewers' top two hitters, Ryan Braun and Prince Fielder, to one hit in five at-bats.
Notes
Cubs OF Marlon Byrd had the day off. … A Cubs fan fell about 14 feet from the lower level in left field trying to catch foul balls during batting practice. Batting practice was delayed about 10 minutes and he was taken to a hospital, but was conscious. A hospital spokeswoman said that privacy laws prohibited them from releasing any more information. … Brewers RHP Jeff Suppan was taken out of the rotation after two shaky starts, including being booed off the field in Friday's 8-1 loss, in favor of Chris Narveson.