There is no viable third party candidate for this upcoming election because of the structure of the electoral college. There is no way for there to be three viable candidates because they would all split the vote and none would have enough to win the election. Thus, voters afraid of their least desired candidate being elected will stick to their party so as not to split their party's vote.
Jill Stien is the closest that any third party canidate has of becoming elected, but she will not. There are not enough people who have heard of her to vote for her. Third party canidtates prove that in part, the political races are really a popularity contest and that sound policies are not important.
There is not a viable third party presidential candidate for this upcoming election. Donald Trump and Hillary Clinton are the only two candidates that have a chance of being elected president. Yes, Gary Johnson might poll higher than recent third party candidates have. However, Johnson does not have the name recognition, money and political infrastructure to run a successful third party candidacy.
No, a viable third party candidate cannot succeed in this upcoming presidential election. I believe that time restraints to gain as much popularity as Trump and Clinton defy the odds of a third party candidate gaining momentum. I also think that the two primary political parties have dominated U.S. politics for too long for a third-party to gain enough recognition.