They probably wouldn't. I think you overestimate the number of people voting Clinton just to stop Trump. A lot of people truly like her the most. (Not me, I'd like to note - not that I will be voting as an Englishman).
It's also worth noting, what third party candidate? There isn't a perfect option there either.
It's a first past the post system - third party candidates are never going to happen because of the psychology, number of ingrained voters, and the way electoral college votes work.
If you are in that situation, the best thing is to vote Clinton, then instantly start putting as much pressure on her and other Democrats to change that system once she is in office. Start now, and get support for a candidate so in the next election you can have one you actually like standing, or at least push towards one. The biggest problem is people getting worked up in the presidential election, then when it's over, ignoring politics for four years.
Change in politics does not happen quickly - generally it involves waiting for old people to die, because people are bad at changing their beliefs. I'd love to believe a third party vote was worthwhile, but the truth is, in reality, it's not. We know that. The system is broken. Work to change that.
First past the post simply isn't democracy. It's a vague semblance of. Until you change that, you'll have two parties and your choice between a turd sandwich and a douche.