Cruz and Rubio would defeat Bernie or Hillary easily (if Trump doesn’t run 3rd party; which he will if he loses the nomination).
Don’t think DJT can do the 3rd party bit - rules in place in many states will not permit that sort of maneuver - or so I’ve heard, which would leave DJT on the ballot in only a few states as 3rd party.
Do not underestimate the Dems, they are staring with almost enough electoral votes to win - only need 100 or less as I recall.