Where can I watch or buy the original 3 hour version?

This is news to me
Hope somebody has an answer would love to see the entire movie

I love MacKenna's Gold, it is one of my two or three all time favorite westerns. I too would like to see the three hour version. My Leonard Maltin movie guide says the film was cut before release, and watching it you can see that this is the case. The gentlemen from town arrive, introduce themselves, and then disappear from the movie. That's unfortunate, because they are played by a half dozen excellent actors. Massey in particular is electrifying in this film and I'd like to see more of his performance. I wonder if a longer version was released abroad (the film was a big hit in Russia). If you ever see Julie Newmar at a film convention (she may be the last surviving member of the cast), and she takes questions from the audience, maybe you can ask what happens in the missing hour. I guess everybody dies by the end.

I recall seeing the townsmen all killed by Indians en route to the canyon.

What I believe was cut were the scenes of The townspeople following Mackenna early on. They just show up in the middle of the movie without much explanation. Given how many stars were among them, it was shocking and in my opinion unnecessary to spend all that money on salaries. They just didn't add much to the story except the general concept of greed

Full widescreen version can be streamed from Amazon.