What an impressive comeback by South Africa in the series! They were trailing 2-0 but managed to secure a remarkable series victory. It's extraordinary, to say the least. The decision of the Aussie captain to bowl first three times in a row after winning the toss is surprising. Australians didn't perform well, and this should serve as a wake-up call for them, especially considering the upcoming World Cup.
Australia were missing some of their star players to be fair(Steve Smith, Pat Cummins etc) and they experimenting with some players which is why they will be a lot, lot stronger in the World Cup.
Also, they tend to perform really well in big events like the World Cup and they are one of the favorites this time around.
Without the mentioned players Australia won the first two ODI matches. Last three matches were really good for South Africa. Australia won the toss in last 3 matches and in all the three matches South Africa is asked to bat first. South Africa used the opportunity well and scored 338/6, 416/5, 315/9 and defended well winning the series. I expect South Africa to give similar performance in the upcoming ODI World Cup and Australia will take it a learning and prepare themselves stronger than before.