As a pretty staunch Clinton supporter, let me just say: I don’t think that a vote for a third-party candidate is a vote for Trump. First of all, it’s at most half a vote for Trump, and that assumes that the person voting for a third party candidate would vote for Clinton in a head-to-head election.
More importantly, though, I think it’s probably a counterproductive thing to say. People want to feel like they have choices, and when you tell them they have no choice, you’re inviting defiance. Remember that infamous Milgram experiment from Intro to Psychology where the participants were asked to send electric shocks to people? Well, Milgram actually tested many variations of how they were asked, and when it was presented as something they had no choice about, the participants rebelled.
This is why I would put the choice to vote third party the following way:
A vote for a third party is a statement that you care more about supporting an alternative viewpoint than about being part of the decision of who the next president will be.
Either Clinton or Trump will be president. That is irrefutable. So you can use your vote pragmatically to affect who wins, or you can vote third party and use your vote for a different purpose.
Personally, I think that this is a very, very high stakes election. Clinton can do the job of being president; Trump can’t. Clinton understands the complexity of the world and of our nation’s problems; Trump can barely form a coherent sentence. Clinton acknowledges and has plans to combat climate change; Trump says it’s a hoax made up by the Chinese. Clinton is thoughtful and measured; Trump is dangerously impulsive. Clinton is a woman, and electing a woman would be an amazing step towards gender equality; Trump is a person who denigrates women and exemplifies every problem with sexism in this country.
Of course I would vote for Clinton regardless, but even if I preferred Johnson or Stein, I would still vote for Clinton because the result of this election would be much more important to me (for the above reasons) than the desire to express an alternative viewpoint. I’d definitely want to express that viewpoint in other ways – maybe volunteer for third party candidates in local elections, discuss their ideas with friends and family, etc. – but I would use my vote pragmatically to influence the outcome of the election.