Sorry if this question has been asked a million times already, but is it ever explained anywhere how Marty finding Doc at the beginning in Pt. 3 affected his influence in reading the letter that was supposed to save his life? If Marty is immediately seeing Doc in 1955 right after he returns to 1985, would he be interfering with his decision to tape the letter Marty wrote him back together that he had just ripped up?
shareI never of thought of this until I saw your post but in this changed timeline (where Marty goes home and the new one comes running up to Doc), there'd be no need for Doc to tape the letter back together to discover that Libyans are going to gun him down because he discovers that he's going to be shot dead in 1885 anyway which means Marty would've just told him.
So after discovering his tombstone, somewhere in between the library scene and the drive-thru scene where Doc has to make that chip, Marty would've just said "Oh, Doc, that letter you ripped up, it was about the fact that you get shot dead by terrorists in 1985 on the night I first went back in time. I'll write you a new one to spell out what you need to do and how you saved yourself last time".
The Back To The Future films connect together but at the end of the day, they each have their own story they're telling, and that would be something that would just break the narrative flow of this film, and even confused the hell out of some people in cinemas who maybe never saw the original five years earlier.