Every report of the sound of gun fire I heard was described as a rapid "pop-pop-pop". That's a select fire 3 round burst if you ask me...which is NOT a civilian AR15.
I've never fired a rifle with 3-round burst capability, but I doubt that it would be described as "pop, pop, pop".
I would expect that the firing rate of the three rounds would match that of the firing rate of an early M-16 on full auto, which is 600 rounds per minute or ten rounds per second. You would have to say, "pop, pop, pop" pretty quickly to say it in one-third of a second.