But it was three rapid strikes by his fellow pace bowler Kagiso Rabada that turned the second day on its head in the final session.
Steyn made a successful return from injury, coming through two of the three games in the series after having been out of action in ODIs for two years.
Du Plessis' comments came after Sri Lanka thrashed South Africa 2-0 in the recently-concluded two-match series, where the fast bowler could only manage to add two wickets in his tally.
South Africa are slated to play two-match Test series, five-match ODI series and one-off T20I against Sri Lanka, beginning July 12 at the Galle International Stadium.
Plunkett has a fair share of experience in the shortest format of the game as he has played 119 T20 matches where he has collected 110 wickets.
Kagiso Rabada, who has been phenomenal this year and looked to be in his career-best form after his dominant performances against India and South Africa, has been ruled out for three months.
The issue was first highlighted when Australia opening batsman David Warner was confronted by a fan and apparently verbally abused as he left the field after being dismissed.
Rabada shared the limelight in South Africa's bowling effort with Morne Morkel, the retiring fast bowler who took his 300th Test wicket on the way to 4/87.
Smith said in the lead-up to the third Test in Cape Town that Rabada's successful appeal against a two-Test ban was "interesting". He questioned why he was not invited to give his side of the story at the judicial hearing.
Rabada was found not guilty of the charge of making inappropriate and deliberate physical contact with a player following a hearing before the Right Honourable Michael Heron QC.
Judicial commissioner Michael Heron, who heard the appeal, said he couldn't be "comfortably satisfied" that Rabada intended to make contact with Smith and found him not guilty of the charge under the ICC's code of conduct.
Kagiso Rabada inclusion in the squad remains stable as no decision has yet been made by the International Cricket Council (ICC).
Rabada was suspended for the remaining two matches of the ongoing four-Test series against Australia after being found guilty of a Level 2 ICC Code of Conduct offence of 'inappropriate and deliberate physical contact with a player'.
Former South Africa cricketer Paul Harris brought Virat Kohli into the controversy, saying the India skipper acted like a "clown" during India's tour to South Africa and yet faced no consequences by the world cricket body for his behaviour.
The Proteas pacer will remain under suspension unless the commissioner permits him to play in the third Test, which will commence from March 22 at Newlands, Cape Town.
Rabada was suspended for South Africa's remaining two Tests of the four-match series against Australia after being found guilty of ICC Code of Conduct offense.
Rabada's heroics helped South Africa defeat Australia by six wickets in Port Elizabeth to level their four-match series at 1-1.
The decision by the International Cricket Council was announced just hours after Rabada was the star of the show in the second Test at St. George's Park.
South Africa clinched its win in Port Elizabeth by chasing down the meagre target of 101 before tea on the fourth day.
The International Cricket Council says it has charged the South Africa fast bowler with a level one offense for screaming in the face of Warner.
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