You have a point:
1st Test:
They played an extra bowler
Steyn has tummy trouble throughout most of it
Between 1st & 2nd
Philander an injury concern
Ryan McClaren had concussion
2nd Test:
Dropped a bowler
Parnell got injured in Aus 1st Innings and didn't bowl in the 2nd
Philander stayed an injury concern right up until the toss
3rd Test:
Replaced Injured Parnell with Abbott
Steyn was injured and probably won't bowl in the rest of the test.
So overall SA have used:
Steyn
Philander
Morkel
Petersen
McClaren
Parnell
Abbott
(Duminy)
(Elgar)
Out of the the 7 frontline bowlers, 3 of them have picked up an injury that prevented them from bowling and/or miss matches, and another was almost added to that list.