choose a class...most 3 gunners shoot USPSA pistol too...the largest class in 3 gun is TacOptics, which is basically limited pistol, limited shotgun (9 rounds no porting or optics), and optics on the rifle...start with a quality belt and underbelt (velcro)...I shoot Limited with a G34 and use a kydex holster on strong hip, two double mag pouches on weak hip... the rest of the gear is modular depending on the match... I use BladeTech kydex for everything, including a couple of AR mag pouches....TecLoc to rear of pistol mags
determine how you are going to reload the shotgun... weak handed is the quickest...I'm old, and set in my ways so load from California Competition 6 round trays strong handed....one fore and one aft of holster... I use a DOH retention holster for both 3 gun and USPSA....3 Gun Gear armband, and side saddle on the Benelli (it doesn't like a lot of weight when shooting slugs), round it out with a 3 Gun Gear shoulder strap to allow for selection of a slugs or birdshot if required.... for a Kyle Lamb match add a Blackhawk shotshell pouch for the 32 round jungle runs....
what pistol do you currently shoot, and what shotgun.. with a 91 clone in .308, the heavy metal 3 gun class is .308/30-06, single stack .45, and pump 12ga...you could shoot single stack in USPSA pistol...