If Hillary insists on running again, I'll have no choice but to believe she's actually an alien from Mars that was sent here to infiltrate and destroy human society.
A frustrated Democrat might have said that to Hubert Humphrey at this time in 1969. It would be very hard for her to win and she would be very unpopular with larger electorate, but still more popular than Trump if his current trajectory is any indication.
She knows that if somehow she gets the Dem nomination in 2020 she once again will be big favorite to be the president. The only thing I think would stop her is if Chelsea wants to go into politics and they decide another Hillary campaign would risk damaging Chelsea's career.
Another factor here, if the third candidate is someone like Cory Booker, Sanders could get plurality of delegates but lose as Booker trades in his delegates for vp slot or whatever he wanted. In that scenario Clinton would be leaking voters from the moment she got the nomination and she would have to spend the general election catering to Sanders concerns.