Not really and not always. The third party vote actually helped Hillary Clinton if one believes the CNN exit polls. Asked third party voters whom they would have voted for if it was just a two candidate race, no third party. 19% answered Trump, 16% Clinton with 65% stating they would not have voted. The later, 65% is a huge number that were driven to the polls so they could vote against both Trump and Clinton. To officially register their anti-Trump, anti-Clinton vote.
https://www.cnn.com/election/2016/results/exit-polls
Being 9 million folks voted third party, that's 5.9 million voters that wouldn't have voted if there hadn't been any third party candidate on the ballot to register their disdain for both major party candidates. Trump would have picked up an additional 1.7 million votes, Hillary an additional 1.4 million. So in 2016 the third party vote actually cost the winner, Trump more than Clinton.
2016 was an abnormal election, an anomaly. Trump and Clinton set the record for the lowest favorable rating of any two major party candidates at 36 and 38% in our history beating out Barry Goldwater who held the lowest favorable record prior to Trump and Clinton at 43%. Trump and Clinton also set the record for the highest unfavorable at 56% and 60%. The only two major party candidates ever to have an unfavorable ratings above 50%. Not even Goldwater back in 1964 broke that mark. His 47% was the highest unfavorable until Trump and Clinton came along. Also have 25% of all Americans wanting neither on to become the the next president is also a record.
What are the odds of the two most despised major party candidates facing each other in the same election. Has to be astronomical.