You're too young to remember Ross Perot's run for the presidency on a third party ticket. He polled very well, pulling enough votes from George H.R. Bush to cause him to lose to (Bill) Clinton. It was a HUGE upset.
This year I think that third party candidates won't stand a chance because the Republicans are united behind Trump and the Democrats are so desperate to defeat Trump so they will all vote for Biden no matter how they feel about him.