Was this originally planned for Eastwood, Van Cleef and Wallach?

Fonda I know was a first choice for Leone. Henry played such goody two shoes characters in his history that Leone was attracted to the audience shock value of him playing a character that can kill a kid.

This is how I understand it from various sources:

Yes, "Once Upon a Time in the West" was supposed to be the fourth and last Dollars movie. It would concern the revelation that everything Il Straniero/Joe/Manco/Blondie/Man With No Name did in the previous movies was all sort of a training for a revenge duel with Frank (presumably because Frank did to him what he did to Harmonica in the finished film). The problem is that on the set of "The Good, The Bad, and The Ugly," Eastwood and Leone had a huge fight that nobody seems to talk about often. I've heard that it was Eastwood finally having enough of the lackadaisical manner in which Italians made movies and I've heard it was Leone finally having enough of Eastwood cheating on his wife with his interpreter. Whatever the subject of the fight was, it is what ruined/ended the relationship between Leone and Eastwood. When it came time to make "Once Upon...", Leone flew to California to personally deliver the script to Eastwood, but Clint would not meet with him.

I've also read an interview with Lee Van Cleef where he revealed he was offered the role of Frank, but turned it down because he didn't like the script (and he didn't like the finished film either when he saw it).

So with his main star gone, Leone was forced to turn Il Straniero/etc into the new character Harmonica and go from there. Leone came up with the idea to have the three gunmen at the beginning of the film be cameos by Eastwood, Van Cleef, and Eli Wallach (though, they were never meant to be the Man With No Name, Angel Eyes, and Tuco--just three randos like in the finished film). It was both a gag and a storytelling device to show the audience, "You thought these guys were bad/cool... Harmonica's even worse/better!" Wallach was in for it. Eastwood was having none of it and Van Cleef turned it down too.

Leone continued to try to work with Clint; he wanted him for the James Coburn character in "Duck, You Sucker," but Eastwood wouldn't do it either.