Plot Hole
So when they first saw the gravestone, it said that Doc was shot down, and it said he had his "beloved Clara". This meant he fell in love with and met her in a very short time before his demise.
Moving on to Marty in the past. The reason Marty and Doc were out on their to horses were because they were surveying the ravine because Marty came back because Doc died in a few days. They realize this woman has her wagon out of control, and Doc saves her before she tumbles down into the ravine. They find out she is Clara, and this is the first time they met. He falls in love with her. Marty also makes he observation that the ravine was called "Clayton Ravine" in his time because Clara supposedly fell to her death there because of the wagon, and that has now changed because Doc saved her. Doc makes the exclamation that this change to the base timeline will have serious consequences to the timeline.
So when Marty saw the gravestone, it implied Clara was saved by Doc, was alive when he died, and fell in love with him as well. Because she was saved by Doc she was able to do so. But Doc was only able to save Clara when Marty went back because he was with Marty looking at the ravine. This does not add up. That meant Marty came back to the past before Doc died and failed, but the creation of the DeLorean was done so that all changes to the timeline happened for the very first time. Clara being alive at he time of Doc's death implied Marty already went through the story of the first 2 movies but failed to save Doc, but then that makes void the first two movies in the first place because it means that Marty is going back for a second time, in which we find out he succeeds in keeping Doc alive.
Am I the only one that noticed this? This is a paradox and makes no sense at all.
If Marty is in the same timespan as when he first went to the past in the first movie in this movie, that meant he couldn't have gone back already to 1885, because going back to 1955 in the first movie showed it was happening for the first time because of the creation of the DeLorean. This makes no sense.