It's not a myth. It's mathematics. We have a winner take all voting system ("first past the post") which means a vote (or ten thousand votes) for a third party who doesn't win is exactly the same mathematically as that number of votes going to the major party candidate
least like the third party candidate, because that candidate's strongest opponent is the one who would have had those votes in the alternative, and the winner is defined as the candidate with more votes than the candidate than the second most votes. The candidate with the third most votes is totally irrelevant until they cease to be the candidate with the third most votes. And they can't do that, because the incentives don't work -- if a candidate from e.g. the Green party could actually win an election, the Democrats would just run that candidate on their ticket, and likewise with the Libertarians and the Republicans. Because running two viable candidates who agree on 80% of the issues against a candidate from the other major party is friendly fire -- if you have a district with 60% left-leaning voters and you run a viable Green candidate against a Democrat and a Republican, the two left-leaning candidates split the left-leaning vote and give the win to the Republican. Viable would-be third party candidates are well aware of this, which is why there are no viable third party candidates -- they have an overwhelming incentive to join a major party as soon as they achieve viability.
This is not an insurmountable problem, you're just solving it in the wrong place. If you want a "third party" candidate to win then what you need is for that candidate to win a major party's primary. What is necessary is for people to stop voting for parties whatsoever and vote for candidates instead.