Broad removed Stiaan van Zyl and Hashim Amla to reduce the hosts to 14 for 2.
Just as AB de Villiers and Dean Elgar appeared to have turned proceedings towards South Africa, Broad captured the wicket of de Villiers with the score on 100.
Moeen Ali claimed the fourth South African wicket.
Elgar, who was unbeaten on 67, will lead the response on Monday.
“It was great to get up to 300,” said Broad of England’s first innings. “We’d have liked a few more but we don’t know how this wicket is going to play.
“Moeen got a few balls to turn so if the pitch continues to deteriorate batting could get harder on days four and five.”
Source: rfi afrique