Ok... the title is click bait... but it is pretty close to accurate.
Here's the deal, in order to win the presidency, a candidate must get 51% of the electoral votes, i.e. 270 electoral votes. The presidency does not go to the candidate with the most electoral votes if none get to that threshold. The House of Representatives chooses the next president. Let me say that one more time... in caps... so that everyone gets it. IF NO CANDIDATE GETS 270 ELECTORAL VOTES THE HOUSE DECIDES THE NEXT PRESIDENT. They have to choose from among the top three of electoral college vote receivers but they make the decision with each state delegation getting one vote. This has happened twice in the history of the US - 1800 & 1824. AND FWIW, no 3rd party candidate has ever won the presidency.
Now, if you believe that Gary Johnson or Jill Stein can get 270 electoral votes... then... don't bother reading this because there is no way to get through your bubble.
If you believe that voting your conscience regardless of any possible outcomes is morally superior, again don't bother reading this because one can't reason with fundamentalists (and that simply is what you are).
Let's play out some scenarios... Here's a ridiculous one but say Gary Johnson unbelievably gets 49% of the electoral votes, Hillary gets 20%, Trump gets 25% and Jill Stein gets the last 6%... Trump likely wins. The House won't give the presidency to a Democrat and that means they'll need to come together. They'd never get all of their states to vote for a libertarian or a Green which leaves Trump. Oh, and by the way, if they can't make a decision, the current Vice President acts as President... giving the office to a Democrat.
Second scenario... Hillary gets 49%, Trump gets 45% and Johnson & Stein split the additional 6... Trump wins for the above reasons. 2/3's of Stein's 3% would have given the election to HRC. It only takes a very small percentage for the third party candidates to be spoilers while at the same time, it is virtually impossible for them to actually win.
You might say, "But the Democrats will get a majority in the House and it is safe then even if the election goes to the House." Well... maybe. Maybe not. Remember, it isn't the membership of the House that votes, it is the delegations and each state gets one vote. So... if the majority of states send a Republican delegation, regardless of the overall balance of the House members, Trump wins. There is no way that a Republican House will elect anyone from another party as president and they have shown they have neither the ability or the patience to thoughtfully compromise. Add that while they're deliberating, Joe Biden is acting president and they will want to put Trump in as quickly as possible.
Just a fun side piece... the Senate chooses the Vice President... so they don't even have to be from the same party.
You might also say, "I live in a safe state so I can vote my conscience without worrying." Again... maybe... maybe not. This is a crazy election. NPR recently had a show where they interviewed a bunch of blue collar Democrats from Monessen, PA who said they were voting for Trump even though they didn't expect he can do any of the things he is promising. Frankly, I don't know what to expect. Hillary could win in a landslide... or Trump might beat her outright. Or the 3rd party spoilers might do just that and the election goes to the House and Donald Trump becomes president. I don't know what will happen and no state feels safe to me at this point. Voting for a third party candidate anywhere just seems too risky to me.
So here's the question... if no third party candidate has a snowball's chance in Phoenix of winning what in the world are Gary Johnson and Jill Stein doing? They have zero influence in the two major parties regardless of how many votes they get. (The people who will have influence in the parties are the ones who do the hard work of organizing within the parties. That is why the Tea Party was so effective in the Republican Party.) They cannot win. They'll never get 270 electoral votes and would never be chosen by the House as it is or will be in 2017. In Jill Stein's case, is she just trying to punish the Democrats for not being pure enough? Is this just some narcissistic game where they can demonstrate their purity and encourage others to do the same, damn the consequences? Or do they really not understand how the process works? Or are they really just plants working for Donald Trump to make him president? Regardless of their intent, if they get enough votes, that is exactly the result of their campaigns.
And of course, this is all just the electoral college stuff... there is the closer to the bone results like Florida in 2000 where if just 30% of those who voted for Ralph Nader had voted for Gore, there would have been no questions regarding who won the state, the SCOTUS mess would never have happened and W would not have been president. You play out the results of that. It is more likely that a third party candidate will simply influence who wins a given state. Remember, every state but two is winner take all in the electoral college so Johnson or Stein could receive 30% of the popular vote and get no electoral votes... but they would influence who wins the various states as per Nader in 2000.
So what does all of this add up to? A vote for anyone other than Hillary is a vote for Trump. He doesn't need to get a majority of the popular vote or even a majority of the electoral college to be the next president. All he needs is to keep Hillary from getting 270 electoral votes... and Jill Stein, Gary Johnson, and all those who vote for either are trying to help him accomplish just that.