“Clerks III” will open in September. The shift just never ends, does it? Weary wage-slave Randal is back, and with all his convenience store cronies, in “Clerks III,” the latest in Kevin Smith’s ongoing Quick Stop saga. The trailer, which dropped today (watch below), features fan-friendly glimpses of original stars Jeff Anderson, Brian O’Halloran, Jason Mewes and, of course, Smith as Silent Bob. A hint to the plot, too, which has Randal — like Smith himself — surviving a life-threatening, life-altering medical emergency. Shocked out of his usual unhappy rut, he’s determined to become a filmmak…