There a sizable chunk of Republicans that don't like Donald Trump, and some Sanders supporters that don't want to support Hillary Clinton. Some of these people, who voted in the Republican or Democratic primaries, are now saying they might vote for a third party candidate -- either Jill Stein of the Green Party, or Gary Johnson of the Libertarian Party.
But they might be disappointed when they see their ballot. That's because those two parties, while the largest of the third parties, are not on the ballot in all 50 states. They are still trying to get on all state ballots, but it is unlikely they'll be able to do so (and neither party was on all 50 state ballots back in 2012 either).
Currently Jill Stein of the Green Party is on the ballot in only 35 states plus the District of Columbia (representing about 425 electoral votes). Gary Johnson of the Libertarian Party is doing a little better, being on the ballot in 43 states plus the District of Columbia. That means voters in 15 states would not be able to vote for Stein, while voters in 7 states would not be able to vote for Johnson.
There's only about 10 weeks left until the election, and states are starting to close ballot access and finalize those ballots. I doubt that either the Greens or the Libertarians will be able to improve their current positions very much.