Trump was smart enough to not run third party in 2016 and if he is not POTUS in 2021, he won’t run third party in 2024 either. He immediately becomes the presumptive Republican nominee.
He didn't have to run third-party in 2016...he won the nomination. But many Republicans have been a thorn in his side since the day he was elected. He could possibly be successful in winning as a third-party candidate.