This one-sided outcome was very unexpected, particularly when the series was in a draw and the two teams had good potential before. The bowling attack of India transformed the game entirely because Arshdeep Singh and Harshit Rana wiped out the top order of the Proteas and left them at 44/5. Ever since, South Africa were playing behind the curve, and despite Aiden Markram and Donovan Ferreira, they were able to only get 117. India did not panic at any point in the pursuit despite the rate of scoring slackening a bit following the sacking of Abhishek Sharma. Tilak Varma cautiously played out before Shivam Dube was sure that he had done things. Generally, it pointed to the strength of the Indian team in terms of bench, but two more T20Is remains and South Africa can still recover strongly.
That was big win of India in important 3rd T20I, which was not looked for as series was on tie. Main reason of victory was fast bowling of India under lead of Arshdeep Singh and Harshit Rana, who soon removed best batters of South Africa, putting Proteas far back at 44/5, and even though Aiden Markram and Donovan Ferreira fought back, the score of 117 runs was not enough. India then chased total well without getting scared. Even after short stop after Abhishek Sharma was out, Tilak Varma was careful and Shivam Dube closed match well, which is example of how good young Indian team is, though, as you say, South Africa can still make return in two remaining games because they surprised everyone in the 2nd match.