A viable third party is literally inviting the Republicans to reign supreme forever
The Republicans are mad happy that the libertarians are so weak.
A third party can be an America First party, not a European-like social democracy/social liberal party.
The way this system is set up is for a two party heads up battle, if we would have a French system which is basically rounds, and in the last round only two parties stand.Then a third or even fourth party would work. And practically speaking that's really what we have now, if Bernie would win the democratic round, and the presidential election. Then like Trump, the party would back him. People forget that Trump was a lot more hated in the GOP than bernie is in the DNC, but they had to fall in line when they realized that this was gonna be their leader.