David Carter, David Lynn and Fredrik Widmark are tied for fourth place at 5-under-par 65 after a day of heavy rainfall.
Forsyth started on No. 9 and picked up birdies at his first two holes. He added a birdie at the 15th and racked up three more on his second nine to post his bogey-free 64.
The Scot, who played in last week's American Express Championship, is currently 19th on the Order of Merit and has aspirations of making the top 15 so he can get an invite to next year's U.S. Open.
'I would be delighted to get into the top 15 in the Order of Merit but no matter how I finished it is still a good year,' said Forsyth. 'I haven't won yet but have had a couple of good seconds, been very consistent and played in two majors for the first time and the WGC-American Express Championship. It's been a big step forward for me.'
Donaldson started on the ninth hole and made birdie. He collected three birdies in a four-hole span from the 15th but dropped a shot at No. 1.
Donaldson once again tallied three birdies in four holes, this time from the second, but fell out of a share of the lead with a bogey at the seventh, his 17th. He birdied the eighth to join Forsyth and Murphy in a tie for first place.
'It was an important birdie to finish, it gives you a big boost to see your name at the top of the leaderboard,' said Donaldson. 'I played the eighth well and finished on a high.'
Murphy, like his fellow co-leaders, got things started on the ninth and went four-under on his round after a birdie at 17 and an eagle at the par-5 18th. He kept the hot streak going as he birdied one and two to go 5 under on the four holes.
He parred his last six to join Forsyth and Donaldson on top.
Carter and Lynn each mixed five birdies, an eagle and two bogeys for their 65s while Widmark recorded seven birdies to only two bogeys to join the pair of Englishmen in fourth place.
Tony Johnstone, Rolf Muntz, Brett Rumford, Stephen Scahill, Henrik Stenson and Steen Tinning share seventh place at 4-under-par 66.
Tobias Dier, the 2002 winner, opened his title defense with a 1-under-par 69 and is part of a group tied for 43rd place. Dier is part of the logjam along with former Ryder Cuppers Barry Lane, Miguel Angel Jimenez and Andrew Coltart.