Question: Back in 1885 why doesn't Doc change the letter he sent to Marty, asking him to bring a can of gas?
Phaneron
19th Aug 2019
Back to the Future Part III (1990)
Answer: This would create a different timeline, not the timeline they are in.
Answer: That would not be possible as in 1885, Doc sent the letter on September 1st, and 1955 Doc sent Marty to 1885 on September 2nd so it was a day later and on the 1st, Doc was not expecting Marty to turn up. However, one CAN ask why Marty and Doc didn't go to the local Western Union office and change it (or write a new one) there since it was in their possession per the gentleman in part 2.
Changing the letter while Marty is in 1885 with Doc would accomplish nothing, because it doesn't it instantly travel to the future. Marty at the end of Part II, for his part, may receive the letter almost immediately, but the letter itself had to wait 70 years to be delivered to him.
I mean, there's no solid rules to time traveling, but just for argument's sake it seems like the letter idea could work... in the franchise, when something is set in motion, the effects usually take place immediately. Take for instance when George and Lorraine kissed at the dance in Part 1. The picture of Marty and his siblings went right back to normal, even though the kids had not been born yet. Doc and Marty changing the Western Union letter "could" have had an immediate effect and a gas can could have materialized in the Delorean, much like we've seen newspaper headlines change before our very eyes, disappearing gravestones, etc.
In your examples, the changes occur to future events. The items that changes, like the picture and newspaper, are from the future themselves. They can't change the past by changing events in the future (like they do in Bill and Ted's). This is why Doc and Marty couldn't go back to 2015 to stop old Biff from taking the DeLorean.
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Answer: When Marty received the letter from Doc in 1955, as seen in the second movie, Doc wrote down that he didn't want Marty to go to 1885 to rescue him because he was happy living in the past. Instead, he wanted Marty to take the Delorean straight back to 1985 and then destroy it so it could never be used for personal gain again.
But once Marty appears in the past Doc could easily change the letter, changing things such that Marty would bring gas with him.
That wouldn't really work with Marty already there. Since Marty and Doc are occupying the same timeline, changing the letter wouldn't do anything until Marty traveled back into the future, at which point the altered letter would be unnecessary since they had found a way for Marty to return.
Phaneron ★
Changing the letter wouldn't have made a difference. When Doc decides to leave 1885, Marty tells Doc that he ripped the fuel line so, with the fuel line damaged and no gas available, bringing a can of gas wouldn't have helped.