--Not two many countries have successful systems with more than two parties. Italy comes to mind as an example of a multiparty system (what is it, 50 governments in 50 years?).--
Italy has endemic corruption which has nothing to do with the number of political parties. That is the principal reason for the large number of "governments" that have fallen due to no confidence votes. In a parliamentary system, a no confidence vote functions as a cleansing mechanism forcing resignation of a Prime Minister and his cabinet and new elections.
If you've got more than two parties, it's difficult for one of them to get a majority. A switch of just a few votes can bring down the government.
Our present difficulties stem from the fact that after 70 years of Dem rule, the GOP is now winning some of the races, and the electorate is split 50-50. I don't know how you solve that problem by introducing a third party.
Yeah, the parties tend to look similar because they are both fighting for that 51st percent of the vote. But as soon as the people make up their mind which direction they want to go--to the left or the right--all of this is going to change.