Anyone disputing the probability that President Obama would beat Trump if he ran for a third term just has to look at the approval ratings. Obama's are 55%, higher than most other presidents leaving office since WWII. Trump's approval ratings on the other hand, are the lowest of any incoming president in close to 25 years.
Moderator's Note: Trump's approval ratings are notably low, but Obama's approval ratings are not especially high - about the same as Bush 41, who was unable to win a successive term. Regardless, any conjecture regarding a third term presidential run is moot.