Ryan McAvoy said:

Okay Tim,

re: my point in t'other thread abot Doc having a bullet-proff vest all along. Is it possible that the Doc we see at the start of BTTF1 had already met BBTF1 Marty and BTTF3 Marty and it was only the appearance of BTTF3 Marty that convinced him to read the letter but he decided to never tell young pre-BTTF1 Marty so the timeline would be maintained?

No, he didn't. At that point, Doc was not wearing a bulletproof vest. However, by the time Marty gets there at the end of the movie, Doc-without-vest never existed, except in Marty's memory. Why Marty's memory seems immutable, while the timeline changes around him, is never addressed. Apparently that's just how time travel works - Your physical body can be affected by change-propagation (see: Marty's disappearing hand), but your memories remain intact, as long as you are a time traveler.

I concede that it makes no sense, obviously.

Also does BBTF3 Cowboy Doc still have the knowledge of meeting BTTF2 Marty in BTTF2 and BTTF3 or just meeting BTTF1 Marty in BTTF1? So is BTTF3 Doc a different DOC than in BTTF1 and BTTF2? Or is BTTF3 Doc the same as BTTF1 Doc but BTTF2 Doc is in another timeline and for our Marty no longer exists?

I need to re-read the plots of 2 & 3 to answer this question, because it's been awhile since I saw them.