We can’t have but two major parties in Federal elections due to the fact that most states are winner take all, and the electoral collage requires an absolute majority less the election goes to the house.
George Wallace won states without a majority of the popular vote as a third party candidate and got all their electoral votes in 1968.
The candidate that wins the most votes in a state will get all its electoral votes and perhaps there would be a deadlock going to the House, perhaps there wouldn’t.
Perot the third candidate for president could have won outright when he rode high in the polls in 1992, then he pulled out of the race for a few weeks and started saying crazy things. He had to tamp down his support so Clinton could win.
Well, we can have more than two, but not for long. The only time we really see more than two gain any ground, historically, is in times of transition, when one party was going extinct and a new one was supplanting it. I don’t think that ever lasts more than one or two federal election cycles though.