The Western Province fast bowler took 4-34 to help dismiss the visitors for 126 in 42.5 overs and secure a first-innings lead of 148. Hardus Viljoen also impressed by grabbing 2-43.
By the close, the hosts had extended their lead to 252 halfway through the series decider.
The West Indies A began the day on 17-1, with Paterson removing Zachary McCaskie (19), Kavem Hodge (nought) and Kirk McKenzie (six) in his first three overs.
The 34-year-old then accounted for Joshua da Silva (five) to leave the visitors reeling on 57-5.
Kevin Sinclair made his way out to the middle and put on 28 runs with Jordan Johnson (33).
Sinclair continued to resist the SA A charge with 50 off 68 balls (eight fours) before falling to Mihlali Mpongwana (1-16).
Ruan de Swardt then wrapped up the innings with a wicket in his only over (the visitors were one batsman short after Jayden Seales injured his shoulder while fielding on day one).
Shamar Joseph (3-26) then dismissed SA A’s top three batsmen – Neil Brand (four), David Bedingham (six) and Raynard van Tonder (30) within the first 12 overs.
Zubayr Hamza (33*) and Khaya Zondo (29*) steadied the innings after tea before bad light brought an early close to the day with the hosts on 104-3.
Photo: Charle Lombard/Gallo Images
