But that doesn't make any sense. Running as a third party candidate means doing a political project in which you're trying to attract supporters to vote for you. If that political project is good, then it's good to vote for, if it's bad, then it's bad to start it in the first place.
If no one voted for a third party, that party would lose relevance and wouldn't be able to accomplish the goal of spreading ideas like you mentioned earlier.
Also, you suggest that once a sort of critical mass of voters prefer a third party candidate, the voting strategy changes and they should vote third party. But it doesn't work that way. How can we tell when we've reached that point, if everyone follows your advice and votes for the less-bad major party? By all appearances, it would seem that the third party has no meaningful support, even if the majority supported it, because they're voting for who they expect to win rather than who they most prefer. For all we know, that could be the situation right now. People can't just all spontaneously decide together to switch, unless you have some means of coordinating it. Enough people have to switch for it to start to seem plausible that it could actually work, and that means those first people would have to act contrary to your rationale.