I see it on fan-made MSDs online, but there are no clear shots of the actual MSD seen in the show that I can find. Memory Alpha has an MSD they claim is based on the one used in the show, which doesn't include the third launcher.
Thing is, contrary to popular belief, today's militaries don't always go into a situation guns blazing, shooting first and asking questions later. There are situations that call for that approach, and even Starfleet has been known to respond in this manner from time to time. But, with today's militaries if armed conflict can be avoided, it is. Defense is still the military's primary nature, even if they find solutions which avoid combat. Aside from technology and the environment in which they operate, there really is so very little difference between Starfleet and modern militaries that this whole "we're not a military even if we act like one. Gene said so" is so ridiculous and suggests the adherents to this thinking understand very little if anything about what a military is and what it does.