The final day-nighter was eventually abandoned under the lights on Wednesday as a downpour ruined the denouement, but Australia could celebrate a satisfying 45-run (DLS method) victory on a day when they were vastly improved after their capitulation in chases in the first two matches.
Half-centuries from Carey (74 off 88 balls) and captain Mitch Marsh (56 off 42), plus 48 from Ollie Peake, helped propel Australia to 9-322 after they'd been sent in, with South Africa never threatening to pull off the highest successful ODI chase at the ground.
By the time the rain, which had delayed the start by half an hour, returned to spoil the fun, Australia were in control, enjoying a much-needed pick-me-up before the Test series starts in Durban on October 8.
Zampa (2-34 off six overs) rediscovered his best after recent struggles and Xavier Bartlett earned a couple of key early wickets (2-25 off five) as only Ryan Rickelton (71no off 81 balls) looked a danger to the visitors, with South Africa limping to 4-145 off 28 overs.
Cummins may have returned as bowler only and not captain, with Marsh completing his ODI stand-in job, but the official skipper's presence seemed to enthuse everyone, with his 0-17 off five probing overs suggesting he's in fine nick to lead his troops in Durban.
"Obviously very pleased. Would have been nice to have had a full game, but the weather gods had something to say about that," shrugged Marsh.
"The way we batted, particularly, showed some real improvement, especially from a few of our young guys, which is always great to see. Disappointing series, but nice to finish with a win."
He again led from the front, smashing his third straight fifty following 60 in Durban and 84 in Johannesburg.
Marsh and Travis Head (24 off 20) were electric again with a 78-run opening blitz at nine-an-over, but this time the middle order built much more assiduously against a Proteas side not helped by resting key spinner and player-of-the-series Keshav Maharaj, and captain Temba Bavuma among four changes.
It wasn't easy once Bjorn Fortuin trapped Marsh lbw and Gerald Coetzee bowled Cooper Connolly (23) with a brilliant yorker, but 20-year-old Peake impressed again with Carey as they put on 73 off 88 balls, the sort of partnership Marsh had been crying out for.
Peake must have thought he'd cracked a boundary off Corbin Bosch to take him to his landmark 50, only for Fortuin to take a dazzling diving catch behind point.
Matt Renshaw (30) and Carey put on 66, but Australia didn't get the acceleration they'd hoped for in the dying overs, with Carey's 88-ball knock eventually ending at the hands of the excellent Bosch (4-60 off nine overs).
It was the keeper's first international half-century in 17 ODI and Test innings since his Ashes hundred in Adelaide last December.
Cameron Green had an eventful time, taking a fearful beamer from Bosch straight into his box, but recovering to hit the next delivery, also a fast full-toss, for six, only to then be bumped out next ball by a short one, seemingly suffering a muscle injury in the process.
He wasn't fit to bowl, leaving an Australian attack already without Nathan Ellis and Jack Edwards even more depleted, but after Bartlett got Quinton de Kock with a ropey ball and skipper Aiden Markram with a beauty, South Africa were soon on the back foot.
Zampa was back to his squeezing, probing best, luring first-match centurion Matthew Breetzke to be stumped on 25 and trapping Tristan Stubbs lbw for 14 off 17 deliveries.
Despite their much-needed one-day victory, Australia were already looking towards the Tests.
"The guys are excited, I know everyone is keen for the Test series," enthused Carey.
"We'll start the prep tomorrow."
