My worry about a third party run would be that more states would enact sore loser laws that would keep him off the ballot. You know Ohio would if Kasich had anything to do with it.
He would have to depend on people writing him in in those states.
Some states already have them and it’s probably too late to get on most primary ballots as a third party.
I doubt the ballot closes in most states by August in a General election year.
If Trump could prove he was cheated out of his nomination, the sore loser laws might not apply.
If he wins 80% of the primaries and still doesn’t have enough delegates locked up, it would certainly toss the Republican process into question.
He could probably challenge the sore loser laws.