Suppose Michael hadn't figured out that Tessio was a traitor, and had been killed on the way to, or at, the Barzini meeting. What would have happened to Clemenza, Tessio and Tom?

Barzini would have eliminated all of them. Clemenza and Tom out of fear of retaliation. Tessio because you can't trust a turncoat.

I think the outcome would be more complex:

Clemenza was loyal to Michael and would never submit to Tessio, much less to Barzini (..."an' I hate dat goddam Barzini..."). He'd have to be killed along with Michael. I'm guessing Rocco, who was loyal to Clem, and Neri, who was loyal to Michael, also would have been whacked. All three were too dangerous to live.

Tessio's regime probably would be loyal to him. But, he might have a problem with Clemenza's regime. If he kept them together, he'd still have to deal with Barzini. I'm sure Tess contacted Barzini first with the offer to set up Michael, in return for being in charge of the Corleone family and a truce with Barz. In doing so, Tess tacitly acknolowledged that Barz would be NY's top Don. I'm guessing, he also had to make some concessions to Barz--like leaving in place the gambling joints Barz established in his territory that he complained about to Michael and Vito. Once started, thos concessions would build. And, Barz would look down on Tess--once a traitor, always a traitor. Barz probably would have demanded to move his own people into Tess's organization--and would have had him killed, in time.

Tom might have come out of it pretty well. Tess would need Tom as a go-between with Michael and Vito's police/political connections. Tom, ever the lawyer, ever the pragmatist, probably would have gone along with that--he didn't have any other choice, really. After getting rid of Tess, Barz probably would have retained Tom as his political factotum because he was useful.

TB, Mafiosi are unpredictable. As Michael told Tom: "... all our people are businessmen.Their loyalty is based on that."