I find it curious how Doc determines what time of day to return to when traveling back to 1885. In part II, Doc, Marty, and Jennifer leave shortly after 10:30am on October 26, 1985 to travel to October 21, 2015. They remain in 2015 for three hours, then Doc sets the time circuits to return to October 26, 1985 at 9:00pm: over ten hours later. In part III, on September 7, 1885 around 8:30 to 9:00am, Doc instructs Marty to set the time circuits to send them to October 27, 1985 at 11:00am.
By traveling ten hours after the time they left 1985 after spending three hours in 2015, they would experience jet lag and would find it difficult to sleep until seven hours after their usual bedtime. It would have made a lot more sense to return around 1:30pm, or possibly 10:45am (since it is harder to adjust to jet lag when traveling to an earlier time zone than a later time zone).
Also, Marty and Jennifer had plans to camp at the lake over the weekend. In part II, Marty and Doc left Jennifer on her porch and then went home. If they arrived at earlier time on October 26, 1985, Marty and Jennifer still could have made it to the lake.
It's also not clear why Doc instructed Marty to enter 11:00am near the end of part III. Perhaps it had something to do with the automobile accident he got into in another timeline while racing with Needles. In part II, Jennifer overhears that Marty gets into an automobile accident with a Rolls-Royce, and near the end of part III, after Marty refuses to race Needles, he looks back and sees a Rolls-Royce that he would have hit if he had raced Needles. This seems to suggest that this was the same point in time that Marty had the automobile accident in the previous timeline.
Perhaps Doc picked 11:00am because it is near the time Marty had the accident. In the middle of part III, Doc mentioned to Marty that he got into an automobile accident. When Marty asked for more information about it, Doc responds to Marty that it might be best if he doesn't know. Perhaps Doc thought Marty might need be provoked by Needles to learn a lesson. Perhaps Doc had hoped that Marty had learned his lesson after accepting the gunfight with Buford Tannen in 1885, and that Marty needed a chance to resist the temptation to accept a risky challenge to prove he's not chicken.