Westwood had been just four shots off the lead at the halfway point but after a one-over-par 73 in the third round he trailed the joint front- runners, Tim Herron and Davis Love, by 10 strokes. He was joined on 212, four under par, by Montgomerie, who fired a 72, as Herron and Love piled on the pressure with rounds of 67 each.
Tom Lehman was one shot back in third place after his third-round 66, while Brandel Chamblee, who entered at the last minute after suffering a back injury three weeks ago, was a further three strokes adrift in fourth after a round of 67.
Ian Woosnam and Bernhard Langer were both way down the field on 216, but at least they were two shots ahead of the world No 1, Tiger Woods, who could only manage a 72.
Love and Herron displayed contrasting games but finished the day as they started it, tied for the lead. Love had eight birdies and three bogeys in a wildly fluctuating round, while Herron carded five birdies in a flawless performance as both completed five-under-par rounds of 67 under ideal conditions on the Bay Hill course.
With hardly a breath of wind, only firm greens prevented the strongest field in the world this year from averaging better than 71.3 strokes.
In Arizona, Britain's Laura Davies saw her bid for a fifth Standard Register Ping title falter when she could only manage a third-round level- par 72 in tough, breezy conditions at Moon Valley, Phoenix, Arizona.
Davies, who set an LPGA record with four tournament wins in a row from 1994, remained the leading Briton - a shot ahead of Scotland's Catriona Matthew - on four under par, but was seven shots behind the leader, Karrie Webb of Australia. Webb upped her chances of a third win of the season with a round of 69 for an 11-under-par 205 overall and a two-shot lead over the Canadian Lorie Kane.
Webb, the US money-leader this year with earnings of almost pounds 200,000, had four birdies as she continued a phenomenal start to the season in which she has never finished lower than eighth in five starts. Last month she won the Australian Masters by 10 shots with a tour record score of 26 under par.
Kane, seeking her first tour win, had a 71, but the overnight leader, Tracy Hanson, crashed out of contention with an 81, leaving her at level par on 216.
Sweden's Annika Sorenstam claimed her first career hole-in-one with an eight iron at the 149-yard second hole. The world No 1 went on to shoot a 71 to finish four under par. Matthew, who finished third in Tucson last week, had four birdies in her round of 70.