The 25-year-old Harmer has been brought into the squad to contest the slow bowler role with leg-spinner Imran Tahir, who recorded figures of 3-108 in 26 overs in the drawn second test in Port Elizabeth.
Harmer, who plays for the Port Elizabeth-based Warriors, is the second highest wicket-taker in South Africa's domestic four-day competition this season with 17 in three matches and is also a handy lower order batsman.
South Africa lead the series 1-0.
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(Reporting by Nick Said; Editing by John O'Brien)
