I haven't seen Shane, but your other two were on my list. The category is "Most Overrated", not "Worst Movies". These movies were overhyped way way beyond their real entertainment value or significance.
Third on my list would be "Out of Africa". It was just dull and pretentious, yet won "Best Film" in 1985.
Now you can say what you like about John Wayne, but some of the greatest actors of his generation, Jimmy Stewart, Lauren Bacall, Katharine Hepburn, Lee Marvin, Robert Mitchum, Henry Fonda, Maureen O'Hara ... etc. etc. etc, ALL said they had never worked with an actor like him. Lee Marvin is one of the great actors of all time, and in The Man Who Shot Liberty Valance John Wayne virtually blows him off the screen. NO ONE has ever had his screen presence. Ever. I love Jeff Bridges, and in every respect I can think of the Coen Brothers version of True Grit is superior to the original except one: John Wayne nails that part and Bridges is a pale imitation.
So I dislike Shane because, quite frankly it is not in the same class as many other great Westerns, and in fact is not even as good as a few modern ones like the remake of 3:10 to Yuma [Far superior to the original Glenn Ford effort, IMHO.] It is merely cited as an iconic film in order to slight the man it does not star. [And by the way, Jack Palance should have gotten the supporting Oscar for that film, so I agree with you, there are great performances in it, and it does not make my "worst" list, by any means... but greatest Western ever? No. It does not belong in even the top ten.]