The point is that South Africa won that 2nd game and therefore they have already won the series. It was a high scoring game and England did a good effort to reach to 325 runs but still lose the match. I am sure that that would have broken their hopes and i am not sure how well they can forget that loss and comeback in this match. Of Course a loss of series by 2-1 is much better as compared to a 3-0 loss and therefore England must have in their back of their minds that they have to win this match and save the whitewash at least (if not the series).
You are right. South Africa won second game and have already won series. Even though England scored much of runs means 325 runs and losing was big hit to their confidence. It is hard to recover after trying so hard and still losing. Now that series is over England main goal is to avoid losing all three games. They will be playing for pride to make final score 2-1 instead of 3-0 whitewash. It is an important match for them to prove they can still compete and finish on high note.