There really is no such thing as a "third party" in our system.
In a parliamentary system, various factions can align, call for an election and choose a leader who represents the interests of that coalition. If the coalition collapses, a vote of "no confidence" can trigger elections, causing a new leader to be selected, representing some new coalition.
In our system, the President is elected for four years and once elected is not going anywhere, short of impeachment and conviction or death. Even then, the successor will be the Vice-President who presumably represents the same interests. Basically our system is a "winner-takes_all" system and the loser gets nothing.
As a result, there will NEVER really be a third party in the US. The third party can never win. If a third party ever did win, it would immediately become the first party.
As a result of this reality, voting for a "third party" is simply the same as staying home and denying one's vote to one or the other of the two actual contending parties.