The problem here is that the formation of a consequential new party is likely to shake-up the other parties, potentially sparking a merger of them, or producing some other configuration of two. Take for example, the rise of the Republicans in the latter half of the nineteenth century. The advent of the GOP led to the final demise of the Whig remnants. The only real chance I see of a viable arrangement wherein there are more than two parties of consequence in the country, short of radically remaking its insitutions, would be to have one party with a hands off approach to social issues (abortion, same-sex marriage, etc.) and a hands-on approach to economics on the left, a hands on approach (or, at least communitarian) on social issues coupled with a hands on approach to the economy, and a third party either taking a hands off approach to both social issues and economics, or which is socially conservative, and very pro-Market otherwise.