There are plenty of parties in the states, nobody votes for them though. Dunno why. I don't see how an entire country can fit its wiews into only two parties.
http://www.politics1.com/parties.htm
In Canada there are 4 major parties, who all have seats in the House of Commons.
Liberal 135
Conservative 99
New Democrats 19
Bloc Quebecois 54
and there is 1 independent seat.
This totals 308 seats.
Also, the Green party has been steadily climbing and may one day hold a seat in the house, hopefully sooner rather than later IMO.
So, to answer your question...isn't there already a third party...more like 50 third parties to choose from?