I think it's not always clearcut, and I do believe that we get mixed points of view.   For instance, the end of ASIB: the scene in Speedy's with Mycroft and John could have been deduced by Sherlock from the conversation John has with him straight afterwards.  But I don't feel the filming shows it that way.  We don't see that scene "in Sherlock's mind", for instance by flashing back to it as John is telling Sherlock about Irene's "witness protection".   So I think we're seeing John's point of view there, listening to Mycroft, believing the story, worrying about how to tell Sherlock.  Sherlock reminiscing with the phone afterwards, is clearly his point of view. 

I do think it's true that you can't divide the perspectives into S1 and S2 are from John's perspective and S3 is from Sherlock's.   There is quite a lot in S1/S2 which is clearly just Sherlock.  There is also some stuff that is clearly just John (I don't go with the explanation that Sherlock could deduce it as it doesn't sit well with me).   Then in S3, although a lot if just Sherlock, some is clearly John.   For instance, John meeting Janine at 221B: even though Sherlock is present for some of that (and could have deduced John's bamboozlement!), I feel we are seeing it more from John's perspective.  

Just about all (if not all) of TAB has to be from Sherlock's perspective, though.  When we see other people's perspectives, that's Sherlock creating and seeing them (for instance the "Watson" scene with Mary and Mycroft, John talking to his maid, John talking to Hooper, etc.).