The way I look at it:

Raiders of the Lost Ark - standalone film

Temple of Doom, Last Crusade, Kingdom of the Crystal Skull - its own separate film trilogy

and:

Back to the Future - standalone film

Back to the Future Parts II & III - its own separate anthology. Though this would be easier if the ending of I didn't spill over into II.

And I still don't get the hate for Alien 3 - I quite enjoyed it, especially the extended version. But Resurrection is a steaming pile of shit.