Du Plessis, who won the toss for the third time in a row and elected to bat, smashed 10 boundaries and three sixes during a third-wicket stand of 112 with JP Duminy.
Left-handed Duminy remained unbeaten on a 34-ball 51 that contained three fours and two sixes as South Africa put on a thrilling exhibition of power hitting in the day-night match.
The Proteas, already enjoying an unassailable 2-0 lead in the series, will look for a clean sweep to make amends for the 1-4 defeat in the preceding one-day internationals.
But the hosts rested their two main players, batsman Kumar Sangakkara and fast bowler Lasith Malinga, and recalled Tillakaratne Dilshan and Suranga Lakmal.
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Du Plessis and Duminy put on their century stand after opener Henry Davids had beeen dismissed off the first ball of the match, leg-before to seamer Nuwan Kulasekara.
Quinton de Kock helped his captain carry the total to 45-1 in the eighth over, before he was leg-before off Ajantha Mendis' second delivery.
Duminy hit his 100th six in Twenty20 internationals when he lofted Kulasekara over the mid-wicket fence in the 17th over.