The 38-year-old, chasing his biggest single payday yet at the $6.5-million event, kept up Friday's momentum as he returned a third round score of 67 for a 16-under-par total of 200.
He might have had a more comfortable lead in search of the $1.25-million first prize, had he not bogeyed the 17th at the Gary Player Country Club.
"I was in between clubs and it put me in trouble but in the end I came out with a good five. I'm still in a strong position and I've got to go out there tomorrow and play really good golf," he told Supersport television.
"It's a tough field, a lead of three doesn't mean much among these guys."
Donaldson leads from Thomas Bjorn of Denmark and American Ryan Moore, who are 13-under-par.
Bjorn played one of the best rounds of the day, birdying the 18th for a second successive day to finish with a 66.
"I got off to a good start and played solid golf. That's what you need to do on this course. If I can shoot 66 or 67 tomorrow, I'll give myself a chance. But you have to play sensible golf, not try and chase down the leader," Bjorn said at the end of his round.
Moore followed a second round 65 with a 67 on Saturday, with five birdies in a bogey-free round.
Spaniard Sergio Garcia and Thongchai Jaidee of Thailand both matched Bjorn's 66 to go 11-under, along with world number three Henrik Stenson of Sweden, who hit a double bogey at the par-five ninth to finish with a 69.
For three-time winner Ernie Els, making a record 18th appearance at the tournament called ‘Africa's major', a triple bogey early in his round saw him finish with a 77 for a seven-over total that leaves him tie bottom of the elite 30-man field.
(Reporting by Mark Gleeson; Editing by Alan Baldwin)
