Third party only splits the vote if a popular candidate abdicates from their party, which as far as I'm aware the only time this ever happened was with Teddy Roosevelt and it effectively handed Wilson the election. This would certainly apply to Sanders if he were to run as an independent. However the examples that get toted as throwing elections like Perot in 1992 and Nader in 2000 weren't actually the case given some of the studies I've seen which indicated that the third party votes were drawn fairly evenly from people who had voted democrat or republican in the previous election with over 60% of the third party votes coming from voters who likely would not have voted otherwise. You can always make the argument that if those voters had been forced to vote for one of the two parties that it may affect the election, but if those voters are choosing to stay home if they can't make that vote it's effectively a moot point. It just gets picked up by the media and propagandized because it threatens the status quo maintained by both parties.
I would like to think this would be a better year to vote third party since Biden's health and some of allegations against him, along with Trump being Trump is a worse match-up than Clinton v. Trump, though I also thought that last time and the third party performance was lackluster, but that also may have been due to the candidates the Greens and Libertarians put up also being relatively lackluster. As far as the leading candidates for the third parties are this year, the ones I've heard is Jacob Hornberger for the Libertarians and Howie Hawkins for the Green Party. One of the comedians I listen to for political news has spoken nothing but praise for Hornberger so there may be potential there. Hawkins has run for governor and other positions here in NY and has some views relatively similar to Sanders. However if the primaries were any indication I sincerely doubt they'd let them anywhere near the debate stage, even if they polled well enough to get there.
Frankly what I dislike is that the polarization the U.S. faces has worked to further entrenched both sides into the view that we'll take anyone from our side as long as it's not the other side. The irony is that if you were to take a decent portion of Sanders voters, non-voters, and people who voted for Trump for sake of voting against the establishment, you probably could create a situation where you could have four viable parties in the U.S. which would do a lot to change the situation were in now for the better since it would likely force candidates to work harder on pleasing their voters rather than their financial backers. Despite what people may say I don't think we will see actual change in the political landscape of the U.S. without having a viable third and possibly fourth party. This election cycle effectively proves that the DNC is always going to go for an establishment candidate. I'd also say, even with including Trump that if you were to look at the last 5 or 6 U.S. presidents, that aside from some differences in social policy and presentation, the overall national situation did not change much if it all based on overall policy. Despite how much the media likes to parrot the DNC taking the high ground, when it's come to policies that have mattered, the DNC has voted right behind Trump (military spending, the present bailout, the patriot act, etc). And if anything Biden's selection proves this, since the DNC made it clear they would take anyone but Sanders, including losing to Trump, though they'll do their best to make it look like they hate Trump and are trying to beat him.