James Bond is the king of recasts Ock. M, Moneypenny, Bond himself, Q, etc. Cameos like the 006 cameo in Thunderball and then Sean Bean in GE, characters being re-written completely, as in Felix Leiter/Sheriff J.W. Pepper/Jack Wade all being the same guy. Some of the people that never were recasted were Tanner, Charles Robinson, and General Gogle (Gogol? I forget). Desmond Llewelyn was Q in 17 of the 20 Bond films, he wasn't in Live and Let Die because they didn't want him in there, and he wasn't in Dr. No because "Major Boothroyd", which is Q's real name, was played by someone else. Now in Die Another Day, Q was played by Johnathan Cleese, aka "R" in The World Is Not Enough
The only one that comes to mind that has recast more times than this is probably X-Men, since their cameos always get recast.
However, I don't believe that after every 3 movies you should recast. I think you should keep original actors as long as possible if they're doing the right job. Dunst I can live without, but I think Maguire is going strong, Franco is great, JJJ is perfect, May is great, Betty is great, Connors/Baker I'm liking a lot, etc. I think they should keep those as long as possible and even Dunst if she can change just a little bit, but once in a while you do have to recast.