Sherlock fakes his death to stop his 3 only friends from being assassinated, but does he explain to John that if he didn't fake his death John would have died?
If I remember correctly, whether Sherlock explained the reason behind him faking his death to John was not shown in the show. It was left for the audience to make an assumption.
The way John reacted when he met Sherlock for the first time after that fake incident, we can assume that Sherlock didn't get the time to explain the reason.
It can also be assumed that, John realises that it was necessary as he knows Sherlock very well and Sherlock won't go to this extent of faking his own death if there was another way out of the situation. Remember, his only question upon seeing Sherlock (and the reason behind all his anger) was, why he was not informed that Sherlock was still alive.