Voting third party is never pointless, your claim is false.

For example, third parties fought for weed legalization for years. My state was one of the first to legalize, which was through ballot signatures collected by Green, Libertarian, and Socialists; not a Democrat amongst them. Every election, third parties would get more and more votes, so now Ds support legalization. Without third party pressure, people would still be serving hard time, the status quo would not budge.

BoTh PaRtIeS want your vote, to vote for a third party is to take a vote away from them. They make changes to their platform to get those voters.

If voting for third parties is pointless, then voting is pointless. There's much more to elections than picking the winning side. It's not about winning, it's about using the political system to achieve your objective.