This match was very competitive, both teams put in a great effort. Now I think England fell far behind due to the poor performance of both the opening batsmen of England, the opening batsmen both performed badly due to which the next batsmen were under pressure. The performance of the middle order batsmen was not bad, they performed quite well. Now the target of 331 runs was quite big, so it can be said that England's bowling line was not good enough. However I would appreciate the South African players, as they won series against strong teams like Australia and England. As South Africa won the series by winning the 1st two matches. I see England more likely to be whitewashed.
Exactly England openers Jamie Smith and Ben Duckett performed poorly which put team under pressure from start. And their middle order including Joe Root and Jacob Bethell and Jos Buttler fought well to get them close to huge target of 331 it was clear that England bowlers had tough day. Your praise for South Africa is well deserved. Winning series against both Australia and England is major achievement and since they have already won series 2-0 your prediction of possible 3-0 whitewash is very plausible and they have ability to do this.