There's really only been one third-party candidate in recent history who made any difference at all, and that was Ross Perot in 1992. He earned 18.9 percent of the popular vote, and if he took that away almost exclusively from GW Bush, it's possible that he handed things to Clinton. In comparison, in 2000, the race was far closer between Bush and Gore, but Ralph Nader took only 2.74 percent of the popular vote. Since Gore won the popular vote but lost by 5 electoral votes, it's not really possible to make a case that Nader blew it for Gore.
Is Gary Johnson any kind of threat? Not really. I'm betting that more people write in Ron Paul than mark an X for Johnson. And it's really unclear where his constituency lies—with liberals who don't like Obama but love a balanced budget? with conservatives who hate taxes but don't mind abortion? He lost my vote when he said "I have smoked marijuana and I have drank alcohol" on NPR. A would-be president should lead the way grammatically.