I'd set up your pistol first.... a good pistol belt, holster and pistol mag pouches...with the .308 gun, the rifle magazines are going to be 20/25 rounders and heavy..... I would go with a 2" belt ... something like an Uncle Mikes duty belt...you will need at least 4 mag pouches for the pistol (and most likely a couple more)....at least 2 mag pouches for the rifle... would look at Brownells for injection moulded gear if you are on a budget... if not, I like Blade Tech formed kydex with teklok attachment...figure out how you are going to reload the shotgun... trays, armband, sidesaddle, etc...

there are some mil surplus mag pouches for the M14.....

if you have never shot a 3 gun match... you really should shoot a couple of pistol matches (USPSA will give you more shots per match)... if you build a modular rig, the pistol remains constant, and you can add the teklok rifle mag pouches and shotshell trays depending on the match...

I started shooting Heavy Metal this year for the first time with a similar set up firearms wise (Colt 1911, Springfield M1A Loaded, and Benelli NOVA). I used a BladeTech holster, and Blackhawk CQC pistol pouches for the pistol, and was pretty happy. I would spring for the m1a kydex or injected molded magazine pouches though. ALOT better than fighting cloth/nylon based pouches. Check out GunfightersINC as he makes some nice M1A pouches:
http://www.gunfightersinc.com/

Also I would look hard at a Load Two vest or armbands/side saddles for the shotgun. Unless you have alot of space on your waist if your a small guy like me the vest allows you more space for your pistol magazines, and rifle magazines since because in Heavy Metal your typically limited to 20 round magazines for rifle, and 10 rounds for the pistol you need to carry more magazines than most people in Tactical would. Another thing I noticed some people do is get a drop leg molle panel, and carry their rifle magazines on their weak side leg which allows them space on their belt to mount shotgun caddies, and pistol magazines on their belt.

Hope some of these suggestions help!