There's nothing in GFIII to say specifically that such a scenario wasn't possible - however, the fact that Michael is tormented by guilt over Fredo, yet simply mentions Tom once or twice without any such feelings (and has no problem conversing with his widow and son), clearly rules out the possibility.

Given that guilt and redemption are major themes of GFIII, a second (third, if you count brother-in-law Carlo) act of fratricide by Michael would have to be mentioned. As it wasn't, it's fair to say it couldn't have happened.