The hallmark progressive achievements made in this country, many that still exist today (to varying degrees ofc) were a result of third party sweat, blood and tears. Literally.

I recommend reading about the social/workers rights movements of the early 1900s. The Progressive Party led by Roosevelt, The Bull Moose Party with social reformers like Jane Addams and Florence Kelly, the Socialist Party of Eugene Debs.... all of these were most prominent in fighting for and ultimately producing a cluster of social welfare, social insurance reforms, women's suffrage, workers rights/5 day work week, etc.

It was the dedication, pressure and will to not fall in line trying to change the two-party duopoly from within but to build their own coalitions, their own movements on the outside, and thus the mainstream parties were eventually forced to inscribe the populus demands into legislation.

All that to say, healthy third parties are a good thing. It builds actual pressure on your legislators. Politicians wont work on your behalf when they know you're voting for them anyway – just line their pockets with money from the bourgeois they actually legislate for. Seeking the change you wish to see via third party can and has produced tremendous gains for the working class.