There is no final to the tri-nations. It's a simple ladder competition. Each team plays 4 matches. Team with the most points wins. South Africa are still well in contention. They only have 2 points (bonus points for finishing within 7 points in each game). Australia and New Zealand both have 9 points.
You get 4 points for a win. So basically, if they can beat both Australia and New Zealand (no sure thing, but i wouldn't put my money against the boks at home) and finish more than 7 points ahead in each game, they will win. If they finish ahead by more than 7 points in at least one of the games, it will come down to points difference between whichever team finished within 7 points in their match. If a team does finish within 7 points though, and South Africa manage to score 4 tries in the match, they'll win.
It's going to be very close and very exciting.