Of course it's not essential, and usually after three, if not even by two, sequels go downhill. I guess for me this is probably the only trilogy that I can't choose which one I like best. It'll probably be just as well to leave it at three, but money talks and this film was such a success, I think at some point there will be a 4th. Whether or not it'll continue to be successful, who knows.
TIS
"Mankind must put an end to war before war puts an end to mankind. War will exist until that distant day when the conscientious objector enjoys the same reputation and prestige that the warrior does today." JFK
It'll probably be just as well to leave it at three, but money talks and this film was such a success, I think at some point there will be a 4th. Whether or not it'll continue to be successful, who knows. 
