The interesting question is who does a third party candidate help or hurt?
In 1992, it definitely pulled votes away from George H.W. Bush, and likely wont he election for Bill Clinton; today, given the fewer people who support Trump over Clinton — but do so more ardently — suggests that Trump would be the beneficiary of much if the mainstream third party candidates (Sanders, Bloomberg, Biden, etc.)
However, some third party candidates are seen as Trump alternatives amongst those who weren’t voting for Clinton anyway: Romney, Ryan, etc. Not that they are likely to run, but a Romney campaign in just the swing states would definitely hurt Trump’s chances.