Each state has a different answer to your question. Above it was explained that Oklahoma limits third-party to specific relationships. Utah does not limit who constitutes a third-party.
For Utah: If you witness acts that were you the victim you would be justified in using deadly force, than you would be justified in using deadly force to defend that third-party. That being said you must still determine if the third-party is actually the victim and not the instigator.
As in almost every case where deadly force is used, you will have to make that judgement as to whether or not it is justified, and be prepared to defend (and live with) your decision.
Utah Concealed Firearms Certified Instructor; NRA-Appointed Training Counselor; NRA Certified Instructor - Pistol, Rifle, Shotgun, Muzzleloading Rifle, Home Firearm Safety; NRA Chief RSO; NRA Certified Pistol Coach; Refuse To Be A Victim® Certified Regional Counselor