Simply stated, if the economy remains relatively strong, he wins 40 Electoral College states against HRC and a bigger number against the rest of the clown car.
If the economy is faltering for a period of six months or more, the Democommie( if it is HRC) or 3rd party candidate has a shot , and lesser if one of the clown car characters, including Biden, who will shoot himself in the foot plenty over the ensuing months.
In an effort to get rid of Trump, if Schultz decides to run as an independent, I can see someone on the conservative side doing the same in the hopes of making it a four way race. That is the way Lincoln narrowly won, and a 3rd or 4th party candidate might see parallels to the Lincoln race. But the bottomline is that Trump is a master at campaigning, and I think whether a 3rd or 4th party candidate run, Trump seizes the day.