If every third-party voter voted for either Trump or Clinton

[°Leprechaun]

Of course, all of this is useless idle speculation, albeit fun for some, no doubt.
IRV IRV IRV
Then it wouldn't be a "what if" scenario... (or approval voting which would have a similar if
not identical outcome).
Write it down 200 times, "I will support IRV". Plurality voting makes not sense to me.
It would be hard to "argue the opposite". The only reason to support it is to maintain
the status quo of "Democrat v Republican" which is a false dichotomy and hasn't seemed to be very effective. Those who support the status quo and think that everything is fine, undoubtedly are more likely to support plurality voting. What happened in Maine (the irv referendum) was a good thing.

[°Leprechaun]

Is this at gunpoint? A large number of people didn't like either of them.

Unfortunately a large number of people also feel that they have to support the "lesser of two evils", even though they don't.

[°Leprechaun]

By the way, Reagan in 1980 didn't win a majority of electoral votes in states where he won a majority and yet it was considered quite a landslide. Most people are not aware of this.