I use the ACE buttstock. You can see it here: ACE Skeleton Buttstock 9.9 Overall Length AR-15 LR-308 Aluminum Black
I wanted a little more neutral balance on my AR. I'm definately not a pro, or even a regular competitor, but the rifle felt a bit light in the muzzle and I like a muzzle heavy feel. This stock gave me that and I like it alot. YMMV
Personally I think that if you are going to run a rifle hard from target to target you want a light front end. Not that a stock has a direct effect here. But (hear me out) the pivot point of the rifle should be some where near the pistol grip. You change that pivot point by adding/removing weight from the front or rear of the rifle. Personally I put a heavier stock on my rifle and it feels much better balanced. My target to target movements and reloading while keeping fire control hand in place are much more natural.
Having a well balanced rifle will be much better then having a poorly balanced light rifle. My .02.
I changed out the stock on my JP CTR to a Magpul PRS. It is heavier then the tube stock that came with the rifle but is balanced much better. Also what I consider a big plus is the PRS fits me perfectly since it is adjustable. HTH
While I haven't dove into 3G yet and therefore am not the best to take advice from (still need a shotty and belt rig), I do have a rifle for it... With a basic A2 stock. I'd recommend an A5 RE/buffer if going adjustable....
I use the Magpul UBR stock. I like the cheek piece that doesnt move when the stock is adjusted. Its also a bit on the heavier side which i like as it keeps the weight of the gun back in my shoulder instead of out on the forend.