As Bartholemew Roberts said, the time window for ballot access is fast closing. Bernie won't run third party, but his followers might stay home on election day, or vote Green. Theoretically, Hillary might patch this up by naming Bernie as VP.
On the Republican side, there's no reason for Trump to contemplate a third-party run as long as he's in the lead for the nomination. But the party might break apart at the convention, with the establishment wing walking out (if Trump looks like he'll be the nominee) and running someone else as the "real" Republican. If the party machinery remains in establishment hands, there could be an issue in some states as to which candidate would appear on the ballot on the official "Republican" line. This might have to be resolved in the courts.
Bloomberg is a factor only in the fevered imaginings of his own mind. I don't see him garnering much support.