So what does a person's skin color have to do with this incident?
Most likely help was on the way. I would have offered my assistance to the officer until his back-up arrived. My weapon would have stayed holstered unless I feared someone life was in danger. Three people acting stupid does not do it for me to draw.
It was once said, "for every action there should be a equal and opposite reaction".
Also from the officers perspective has does he know you are not some crazy guy just like the three he is dealing with. I suggest you always ask the officer before offering any assistance. Perhaps if you were to witness this in the future you first action should be to call 9-1-1 to make sure the officer has help on the way.