What do you think voting is doing if its not increasing or decreasing the likelihood of a candidate winning?

Establishing a credible threat of defection in response to unacceptable policy. Building up a party that actually represents my interests.

If there’s only two possible outcomes between three choices, and one of those choices is clearly the worst outcome and another one of them is clearly not a possible outcome, which choice would you make and why?

That question is much too abstract.

A third party winning this election is not realistic, but there are other tactical and ethical reasons for voting for them that have nothing to do with winning, as I said.