We have viable third/fourth parties at the national level, and usually they get a significant number of seats (even though they don't form a government). This election was an exception, because of the polarising actions of the U.S. president and the perception that the Conservative leader was too closely aligned with him, but even now, PM Carney has to cooperate with them.
Imagine how different U.S. politics would be if a third party held even 5–10 seats in the Senate.