You could pick almost any starting XV from the Lions squad, and I don't think it would make much difference quality wise, they are all experienced internationals.
The issue is mental, rather than physical. In that second test, they probably thought that the Boks would fade in the second half, like they did in the 'A' game and the first test, and going into the break already leading, the Lions players probably thought that they just had to go through the motions to claim the win.
As a result, the intensity dropped off, and SA had all the momentum.
So the attitude for Test 3 is that every player must give 100% effort for 80 minutes, or however long they are on the field. Do that, and we're in with a shout.