This will be a set up for 3 gun Comp.  I need to hold 2-4 AR mags, 12-18 12 ga Shotshells, 4-Glock G17 mags. Also need to tie my Holster into this.   Here is what I am thinking right now.

SKD Eagle Chest Rig as seen in EE for 75.00

Tactical Taylor 6 round MOLLE shot shell holders x3

Blade-Tech Tactical Thigh Holster for the Glock G17

Dont I need some kind of belt to tie all of this together?  Will this sett up work for what I want to do?  And anyone suggest a Belt?

For a comp you will have to figure out what will make you perform your best.

For Me I'd probably start with a chest rig with 4 open top M16 shingle type mag holders with the shotshell holders on the front. This is worn high on the chest, on maybe a one piece TT MAV.
Pistol Mag holders go on belt, opposite side to the front, of a drop holster. Holster would Probably be a Safariland, for the retention hood. Button thumbbreaks are a PITA for constant use. Stiff duty belt with the inner velcro belt, that you actually run thru your pants beltloops.

This would be a different setup than my training/range gear layout.