The problem with the 3rd parties typically is not their candidates (granted, some are better than others just as some major party candidates are worse than others, so that's not a blanket statement). Rather, the bigger problem is the witless sycophants.
As I've posted previously, eligible voters are roughly broken down as follows:
20% who would vote for Charles Manson if he were the R nominee
20% who would vote for Charles Manson if he were the D nominee.
40% who won't vote no matter who the R's or D's nominate.
20% who aren't aligned with a party, and are responsible for actually choosing the winner.
What that means is that if even 75% of that final group were to rally around a single candidate other than one of the Manson conjoined twins, that candidate would still only get around 24% of the vote, and each of the conjoined twins would get around 36% (give or take a bit). But it would still be that final 5% that chose the winner, which could only be a Manson twin.