Los Angeles, the eighth and last playoff seed, took a 3-0 lead in the Western Conference semifinals and can finish off a sweep at home Sunday. The Kings have outscored the Blues 12-5 in the three games after knocking off top-seeded Vancouver in the opening round.
Jonathan Quick, who had a 4-8 career playoff record coming into this season, has won seven of his eight post-season starts in 2012. Quick made 18 saves Thursday.
Stewart's goal came 1:13 into the middle period and it took Los Angeles just 40 seconds to regain the lead on a wrist shot by Dwight King.
Richards extended the Los Angeles lead while Jamie Langenbrunner was serving a 2-minute penalty for interference.
After Stewart brought St. Louis within a goal early in the third, Drew Doughty restored some insurance for the Kings by scoring on a slap shot with 11:48 to go in regulation.
Brian Elliott, who led the NHL in goals-against average and save percentage during the regular season, gave up the four goals on 22 shots.