SUN CITY, South Africa, Dec 1, 2012 (AFP) - Consistent play the past three days gave Kaymer the lead after he finished with a five-under-par 211 at the 7,162-metre Gary Player Country Club.
The former world number four started the day badly with a bogey on the par-four third, but recovered well with a steady performance and carded three birdies.
The South Africans are breathing down his neck, though, with 2010 Open Champion Oosthuizen just one stroke behind and 2011 Masters winner Schwartzel trailing by two.
Oosthuizen shot the best round of the day with three-under-par 69, thanks to three birdies, despite bogeys on the fifth and tenth and several missed putts that could have won strokes.
The highest-ranked of the 12 elite players, Englishman Justin Rose, also hit 69 in his best round yet in the tournament.
The world number four carded a 10-stroke improvement on his Friday performance of 79.
Defending champion Lee Westwood remains in the running for a record third consecutive victory at the African 'major'.
The Englishman is tied with American Bill Haas at two-under-par 214, trailing leader Kaymer by three.
Round two leader Scotsman Paul Lawrie started the day with a bogey on the first hole and slid further back with bogeys on the third, fifth, eighth, eleventh and thirteenth to card two-over-par 75.
Scores after third round of Nedbank Golf Challenge invitation tournament at par-72, 7162-metre Gary Player Country Club Saturday:
211 - Martin Kaymer (GER) 72-69-70
212 - Louis Oosthuizen (RSA) 71-72-69
213 - Charl Schwartzel (RSA) 72-71-70
214 - Bill Haas (USA) 70-73-71, Lee Westwood (ENG) 71-73-70
215 - Paul Lawrie (SCO) 71-69-75
218 - Peter Hanson (SWE) 72-73-73
221 - Francesco Molinari (ITA) 72-71-78, Carl Pettersson (SWE) 72-75-74, Justin Rose (ENG) 73-79-69
222 - Nicolas Colsaerts (BEL) 70-78-74
223 - Garth Mulroy (RSA) 75-73-75