If I read it correctly, if South Africa will win the series with 3-0 they will be the number one in Test matches. But a lot of cricket has to played yet, before that to happen.
This is an interesting fact:
'The heroics from Duminy and Steyn ensured that Australia conceded the first-innings lead in Melbourne for the first time in 12 years. The last team which managed this was West Indies, in 1996, when they bowled Australia out for 219 and then scored 255 in a match they eventually won by six wickets.'
Cricinfo - Australia get a taste of their own medicine