I agree "third parties" as the Presidential level act more like "spoilers" than "winners".

But as you point out, third parties can and have been quite suceesful at the local level and for most people local politics are more impactful to their daily lives than national politics.

I think a moderate third party could do very well at the local level in progressive liberal states among moderate Democrats that can't bring themselves to vote Republican.

The same could be true in some conservative states where a coalition of moderate Democrats and moderate Republicans might win against a very conservative Republican.

Overtime, if these politicians that build a strong local reputation like socialist Bernie Sanders did as mayor they can run for statewide office.

Even half a dozen independent or 3rd members of the House or Senate could wield extraordinary influence (look at the influence Bernie Sanders yields) given how evenly divided this country is.