#3 He gets thrown out of the faction
While this is the unlikeliest of all outcomes, it is the most logical one. Never has the integrity of The Shield been doubted so much. These three were supposed to be the guys that would always have each other's backs.
Now cracks seem to appear. Ambrose doesn't seem happy, the team isn't cohesive enough, and there just isn't enough space for so many top babyfaces.
At the same time, The Shield isn't about loyalty, it's about dishing out justice. Rollins and Reigns should logically get tired of the tease and unpredictability of Ambrose betraying them. To put an end to this with finality, they may chose to throw him out, maybe replace him with someone, say Finn Balor.
This would mean turning the two top faces of the brand heel, it is the most unlikely outcome.