I have a '97 C230 w/o bose, but the speakers are the same configuration in the rear. I've always wondered why there are three. If I remember correctly though, the numbers on the back of the speakers are the same for two, but different for one of them, suggesting it's a different type of speaker. My guess is the extra speaker is specifically for bass?
But, I figure the hole for a fourth speaker would be a great bass port should I decide to buy subs.