A quick refresher on 3gun shotgun rules:
Open Division - Gun can hold up to 11 rounds. Porting, muzzle brakes, and optical sights are all allowed, although many shooters just run a bead. The optics setup to beat is a Docter or JPoint in a low barrel mount. Speedloaders or detachable magazines are allowed, and are really necessary to be competitive.
Tactical/Limited Division - Gun can hold up to 9 rounds. Iron sights only, most people use a fiber optic bead. No porting or muzzle brakes. No speedloaders or detachable magazines.
In either case, the gun must be able to hold the maximum number of rounds allowed under the rules, in order to be competitive. The gun must also be completely reliable under all conditions.
Which brings us back to your specific question. My answer probably isn't going to make you happy, but here you go - the first thing you should do is sell your 11-87 and buy a Benelli M1 or M2 or an FN SLP. The Remington 11-87 is not reliable enough to handle the high round counts and rapid shooting found in 3gun. It is arguably reliable enough for use as a defensive shotgun, but I personally have seen too many fail to trust them.
Apart from that, your setup depends on what division you want to shoot in. If you want to shoot open, I'd suggest having Benny Hill or another gunsmith familiar with multigun shooting build it for you. You can probably build a Tactical/Limited shotgun yourself, by adding an 8-round magazine tube, fiber-optic bead (I'd recommend against rifle sights unless you can fold the rear ones out of the way,) and an oversized bolt release. That's all you really need, everything else is a comfort item.
- Chris