If Trump (as is likely) is the candidate of the Reps and Bernie (as is unlikely) is the candidate of the Dems there will clearly be at last one and likely two third party challenges.
Some on the left claim that Trump will beat Hillary because she lacks a dogmatic progressive message. This is the mirror image of the Cruz campaign that claims the GOP can only win if it becomes the Tea Party.
The lefties are wrong. Bernie can not win, but the right could have the correct idea.
If it is Trump (R) v Hillary (D), ALL of the Wall street corporate money will go to her. There is no credible left third party challenge as Bernie will at least support her and likely stand beside her as a surrogate. There are credibke right wing groups .. e.g. Adelson .. who would support an anything but Hillary or Trump push. They have more than enough money to support a right wing third party, in the manner of TR’s Bull Moose Party. There are also enough 
“Republican” Tea Party fanatics who might be wiling to head such a ticket.
This “True Republican Party” would run with commitment
to join with the Tea Party caucus in the Senate and House to elect true republicans. They would join with the RNC to support local races.
The goal would be to have a non majority election that might well end up in the GOP controlled House!