Of course, the Canadian elections this year were proof that it CAN happen - but we should probably still give it an election or two before concluding the NDP has definitively claimed its spot among the big two. And last year's British elections were an indication of the opposite - the Lib Dems were scoring rather well in the polls all throughout the campaign, then when it came down to it they even lost votes and seats. Britain has had more than two parties for centuries, and in all that time, only once did the third party usurp the spot of the second - and that was only after part of said second party had defected to their main rivals to put those in power for a generation. Coalition governments haven't been much more frequent, except during wartime. Being the third party in a FPTP voting system is very, very hard. Especially in FPTP voting systems that have features that just take for granted there will only ever be two parties, like the US House and Senate do.

A third party candidate winning the presidency is not inconceivable - a number of people have come relatively close, and some independents have won governor races - but a genuine nation-wide party with representations in the House, Senate and state/local legislatures is an entirely different story.

Yes. If it ever happens.

Defection of sitting legislators on a large scale is pretty much the only way I can see it happening, frankly.