Sure on the national level that’s true, but on the local level third parties can definitely make big gains in areas that are safe democrat seats traditionally. Then from there you fight to implement those kinds of systems at the local level and work up from there.
If you’re interested in that you can always check out the DSA and see if they have a local chapter in your area. They’re not a political party right now but imo if anything is gonna form a true labor third party in the US it will most likely come from there.