Sent in to bat, South Africa struggled to post 136/9 from their 20 overs, a total that New Zealand chased down with ease, losing just two wickets and with 22 balls remaining.
The top six combined for just 68 runs before leaving the lower order to try and muster a total, with seamer Nqobani Mokoena ultimately top-scoring with a quickfire 26 at the tail-end of the innings.
All of New Zealand's bowlers claimed wickets, with Ben Sears' 2 for 27 arguably the standout. Mitchell Santner (2-21) and Lockie Ferguson (1-9) also made significant contributions.
South Africa never threatened to make a game of it in the field. Tom Latham led the way for the Kiwis with an unbeaten 63 off 55 balls, putting on 96 for the opening wicket alongside South African-born Devon Conway, who made 39 from 26 before being caught in the deep off spinner Keshav Maharaj - though the match was all but over as a contest by that stage.
The fourth game of the series will take place in Wellington on Sunday. South Africa have rested most of their regular stars, with only three players who appeared in the recent T20 World Cup included on tour.
Top stars like Quinton de Kock, Aiden Markram, Heinrich Klaasen and Kagiso Rabada are all absent, with the touring party consisting of a number of emerging talents.
Proteas coach Shukri Conrad says the "jury is out" on whether the T20I series in New Zealand should have been given international status but insists the tour has been an "invaluable exercise".
The Proteas levelled the five-match T20I series against New Zealand in Wellington on Sunday, securing a tense 19-run victory in the fourth match to set up a decisive final clash in Christchurch on Wednesday.
South Africa's batting coach Ashwell Prince said his team "didn't get any rhythm" after the Proteas were bowled out for 107 in a seven-wicket defeat to New Zealand in Hamilton, leveling the T20 series 1-1.
The Proteas crumbled with the bat to go down by 68 runs against New Zealand in the second T20I in Hamilton on Tuesday with the five-match series now level at 1-1.
