Clark was joined atop the leaderboard by second-round leader Maarten Lafeber at 15-under-par 198. Lafeber posted a 3-under 68 in the third round.
Alastair Forsyth carded a 4-under 67 to join Angel Cabrera in third place at 14-under-par 199. Adam Scott (64), Darren Clarke (69) and Thongchai Jaidee (66) are two strokes further back at minus-12.
Clark got off to a hot start as he rolled in an eagle putt on the par-5 third. He moved to 12 under when his birdie putt at the fifth found the bottom of the cup.
The South African made it two in a row with a birdie at the par-5 sixth. Clark ran off six straight pars around the turn from the seventh.
The 29-year-old birdied the 13th and joined Lafeber in the lead at minus-15 with a birdie on No. 14.
However, Clark tripped to a bogey at the next. He bounced back to birdie 17 and after a par at the last, he shared the third-round lead.
'This is my fourth year in a row here and every year I seem to have a good tournament,' said Clark. 'I think the course is a lot firmer than it's been before and that's allowed the tee shots to go a little further and make the par-5s more reachable.'
Lafeber matched Clark's quick start. He also eagled the third to move to 14 under. The Dutchman came right back with a birdie from 12 feet out on No. 4 to take a two-stroke lead over Clark.
The 30-year-old Lafeber faltered to just his second bogey of the week on the par-3 eighth. After five consecutive pars around the turn, Lafeber got back to minus-15 with a birdie on the par-4 14th. He parred out to finish there.
'I am happy in this position and that was my goal, to still be leading after three rounds,' Lafeber said. 'It's very satisfying to have done that, but there are a lot of strong players behind me, so I have to play really well tomorrow to finish on top.'
Luke Donald and Nick Dougherty both shot 5-under 66 on Saturday at Loch Lomond Golf Club. They were joined in a tie for eighth place at 11-under-par 202 by Miguel Angel Jimenez.
World No. 3 Ernie Els opened with a bogey and closed with a double bogey to finish off a round of 4-under 67. He is one stroke further back at minus-10 alongside 1999 champion Colin Montgomerie.
Thomas Levet, the defending champion, is well off the pace after an even-par 71. He stands at minus-3 in a tie for 52nd.