So the way I'd see it working in the US would be that a third-party candidate wouldn't win anything but would instead take away votes from the main two parties, mainly among the independent voters. In order to reclaim those independent voters, the two main parties would then need to run better candidates to win those voters back. As of right now both parties run mediocre to awful candidates and unless you're really on board with one of the parties, you just pick the one who's the least worse.
Again, I don't disagree with the sentiment, and I'm not expert in the US democratic system, so correct me if I'm wrong but the way I see it, is that should a third party gain enough votes to make a difference, it's likely they'd have a couple of seats in the house? With that level of power they could agree to vote inline with D or R, and effectively swing a bill from pass to fail, or vice versa, such is the closeness of your house votes.
We had a similar thing here where a party that only received 0.9% of the national vote entered into an agreement with the largest party in order to take them past 50% of the seats required to have a majority. This was a fairly spectacular **** up for uniquely British reasons, but it demonstrates what the next step would be for a third party in the US - this party got a Billion quid for their manifesto pledges and had a deciding say on some of the biggest 'constitutional' issues of our lifetime (in the UK), for 0.9% of the vote! Even if it only gave the third party the guaranteed support required for one key issue bill, it could be seismic for the supporters of that party. That said, there's no longevity provided to the smaller party, come the next election, a small shift one way or the other could render them irrelevant again. To try and draw another parallel with the UK, in 2010 the third party got into government as part of a coalition required for a majority. One of the issues their voters responded to was the equivalent of student loan forgiveness. That manifesto pledge was ignored by the larger party, so despite getting into government, that party was never able to come good on its pledge, and come the next election its voters lost faith and their vote share declined by 65% and did not recover - the larger party remained in government.
The effect of a disruptive third party is a double edged sword, but it does not last if the system is geared towards winner-takes-all. I agree with your stance if what you are voting for were a simple matter of competing ideologies that are both/all true from a certain point of view, but Trump getting in isn't just about a simple 'return' to conservative or republican ideology, it's a rubber stamp of approval for all the **** Trump and his allies want to inflict on America likely for their own personal gain.
Generally speaking I like most of the US presidents I've seen in my lifetime, If I hadn't looked it up one time I wouldn't be able to tell you which were Red and which were Blue. Trump is a nob, he doesn't deserve to have a say in your government irrespective of the political leanings of his party and it's a sad fact that ballots have to be cast to get him out, but it's fact nonetheless. That is an issue worth sacrificing ideology for, in my humble opinion.