People here have a hard time understanding the role that context has played in Cap/Bucky vs Iron Man fight.
Ok now lets change a few details in the same fight , suppose Iron Man is really angry and wants to kill them both , at the start of the fight when Iron Man is standing next to cap , he will use his lasers to cut his head , then he will go after Bucky and cut his head too ! Just like that , no muss no fuss.