Three or more would be good.
Three parties are bad because one becomes basically a whore, trying to steal from the other two larger parties. And yes, it's stealing.
The best example of this are the Liberal Democrats here in the U.K. They try to funnel votes away from which ever party is looking weaker at a given moment. This means every couple of years thier ideology completely morphs. It also means that when they finally do get power, they will no longer be able to keep any of thier promises, because by doing one thing they upset a large portion of thier party. They are basically taking votes away from real parties until they gain power, when they will inevitably split into other parties, sending the party system into chaos, or become totally ineffective, equally damning.
That's why I support four or even five parties.