Since there are five eligible candidates this year, there will automatically be three nominees. A fourth is possible, but only if there's a close race between two contenders within three votes.
I agree with the consensus that the race is likely coming down to Cats: The Jellicle Ball and Ragtime. While I can see the latter benefitting from how differently it hits now given the show’s timely nature, I'd still give the edge to the former. Especially since it does fit the mold of some of the more recent winners of this category as this big reimagining like The Color Purple, Once on This Island, Company, and Sunset Boulevard.
As for what takes the third slot, early reports for Rocky Horror suggests it at least has potential acting nominations for Luke Evans and Amber Gray. Chess does have potential acting noms for Nicholas Christopher and Lea Michele, but I’m not sure if either could make the cut.
Gold Derby currently lists only eight eligible candidates for Best Lead Actor in a Musical. Unless there ends up being more, there will automatically be four nominees. A fifth is possible, but only if there’s a close race between two contenders within three votes. If that category only has four this year, I can see the lineup consisting of Evans, Henry, Tutty, and Uranowitz, but no Christopher. As for Michele, a Best Lead Actress in a Musical nomination for her is gonna have to depend on how the nominating committee feels about Chess overall. Especially since there are people out there who feel her singing in the show is great, but not so much her acting.