In a number of states, you cannot run in the election as a third party candidate if you lost the primary for that state. I’m not sure how widespread that is, but it’s come up in several recent elections. I think they’re even called something like anti-spoiler rules.

I think there’s pollsters who are looking at three way races now (usually throwing in Kennedy), but I don’t know if they’ve been able to say whether he pulls more Trump or Biden voters yet.