In the UK, the Lib Dems have decided which of the ‘big’ parties sits in government and which in opposition. The Bloc Quebecois is one of the major parties in Quebec. In India, the two biggest parties get 50-60% of the total votes polled, and most governments are composed of multi-party coalitions. Also about a third of states have governments led by a third party.

I am aware. But that doesn't really change what I've said. You're comparing smaller elections for seats with a big election like the U.S. president. Those elections still have 1-2 dominant parties, etc.

Third parties cannot win only when everyone thinks they can’t win.

You can't just wish away the spoiler effect.