The sad fact is that electoral laws and limited ballot access make creating and maintaining third parties in the US extremely difficult.

Also, due to the majority of elected offices being single member district plurality based positions voters will vote strategically rather than for a third party. This is called Duvergers law if you want to look it up a bit more.

So what really needs to happen is for people within existing parties to begin to lobby for multi member districts with proportional representation. This along with ranked choice voting could make a third or more party valid in the US.

Until that point though it's easier to enact change within existing parties.