I dont wanna type a long reply, but IMHO the easiest way to get a 3rd party in the US is:
Run a 3rd party in the few states with RCV for House seats (Maine, Alaska). Promise to caucus with whichever party would otherwise win the seats there on important issues. Currently that would give you 3 reps iirc.
Cross your fingers and hope for a divided house.
This tiny party would now be a kingmaker. It could make legislation aiding PR/RCV the primary condition of its support. Namely repealing the Uniform congressional districts act which currently prevetns any state from adopting PR.
I dont wanna type a long reply, but IMHO the easiest way to get a 3rd party in the US is: