Barry Painting posted a 67 to share second place with Desvonde Botes (68) at 8-under-par 208. Louis Oosthuizen carded 68 for the second consecutive day to climb into fourth place at 6-under-par 210.
McLardy led by just one stroke to open the round, but birdies at each of the first two holes at Leopard Creek stretched that cushion to three strokes.
The South African faltered to a bogey at the par-4 fourth, but came right back to birdie the very next hole.
Around the turn, he dropped in back-to-back birdies from the 11th to extend his lead. He converted a birdie at the par-5 15th for the third straight round and closed out his scoring with a birdie at the par-5 closing hole.
'A seven-shot lead is great but 17 would be better,' said McLardy. 'A guy like Desvonde is capable of shooting 30 on the first nine and you can shoot 35 and feel like you've played well. You haven't done anything wrong but suddenly the tournament's on.
'The routine will stay the same. My confidence took a bit of a knock for a long time, but I've got a lot more self-belief now. I felt very confident over every shot today and my swing felt good.'
Painting opened his round six shots behind McLardy. He tried to keep pace with birdies at the second and seventh, but was derailed by a bogey on No. 8. He closed strong as he ran off four straight birdies from the 14th to move into a share of second place.
Botes managed just one bogey and one birdie over his opening nine holes. He dropped another stroke at the 10th, but burst up the leaderboard from there. He drained a birdie on 13 and converted an eagle at the 15th before closing with back-to-back birdies from the 17th.
Craig Lile, who battled nausea and dehydration on the course during the second round, stumbled to a 3-over 75. He shares fifth place at 5-under-par 211 with Thomas Aiken, who posted a 5-under 67.
Anton Haig, James Kingston, Keith Horne, Alan McLean and Nic Henning share seventh place one stroke further back at minus-4.
Ashleigh Simon, the first woman to play in a men's event in South African Tour history, continued to struggle. She posted an 8-over 80 to fall into last place at 21-over-par 237.