A way to make them all fit together...

The first two didn't really fit together. Mainly in the fact that Josiah Gordon "Doc" Scurlock (play by Kiefer Sutherland) lives in the first film and tells the story of Billy the Kid. While in the second one he dies in a gun fight and Billy outlives him.

A way that the films (even the upcoming one) could fit together is if they were looked upon more as folk lore stories, being told by different people, who didn't know all the details, or the tellers were so old that their memories had faded and were (in places) just flat out wrong. This would clear up all the problems between the first two and make it so they could bring back whomever they wanted for the 3rd one.