Meh that's like saying voting for sanders isn't voting.

I couldn't disagree more. Sanders is running in the Primary of one of the Two Parties. If he wins their nomination, he becomes one of the Two Parties.

Third parties cannot win in a First Past the Post system. It's not a low chance. It's zero chance. No third party has ever won in a US Presidential election (discounting party system changes, which aren't actual third parties).

See https://en.wikipedia.org/wiki/First-past-the-post_voting#Tactical_voting

Its a BS argument that the mainstream parties use to discourage you from voting for someone that actually represents you interest.

The exact opposite. Trump wants you to vote Green, to take votes from Hillary. And Hillary wants you to vote Libertarian, to steal votes from Trump. The Two Parties want you to vote for the third party closest to their opponent. If Bernie ran on a third-party ticket, he would only steal votes from Hillary, and throw the election for Trump. That's how First Past The Post works.

If your slightly less shitty mainstream candidate is at risk of loosing because you voted for a 3rd party then you are going to force their platform in your direction

That's the sole argument for voting third party instead of mainstream, in FPTP: the nuclear option. If you're willing to, for all practical purposes, vote for Trump, in order to encourage Hillary more to the left, you can Press the Red Button. But the net effect is the same as not voting. So, as far as "pushing to the left" goes, there's no difference whether you vote third party or don't vote.