The odds of you vote changing that is tiny. IMO, convincing others to vote for a 3rd party is a bad idea, but voting for them is just as 'meaningless' as any other vote in terms of changing who win. However, the power of voting is in the message not just who wins.

Defacto third party's scare the primary party's so they are quick to gain the talking points as soon as those party gain any traction. Most recently Libertarian message was somewhat co opted by Tea Party Republicans. Though in a very mutated form that for example ignored their drug stance.