I've discussed this previosly, under our winner take all syhstem, it's not practical to have more than two parties nearly ever - for a third party to succeed, it generally is to one side of one of the two parties, and splits the vote of the party it's closest to, and pretty much always thereby helps the party it's furthest from win. This is the case with Gore, with Perot, with Teddy Roosevelt, and so on.
For a third party win is so nearly impossible, it's have to not only pull away enough of the party it's closer to, but enough of the second party as well to get a majority.
As I've said before, what we should do is for states to enact ranked voting, where you can vote for first choice, second choice, and so on and have your vote count even if first choice doesn't win.