There is no definitive answer to this question since different organizations that host 3-gun competitions have different rules regarding what firearms are allowed. However, in general, most organizations do allow pistols grips on shotguns as long as the shotgun meets certain other criteria (e.g., overall length, minimum barrel length, etc.). So if you’re thinking about using a pistol grip shotgun in a 3-gun competition, it’s worth checking out the particular organization’s rules to see if your gun will be allowed.
Pistol grips can give you an advantage when shooting a shotgun, particularly in terms of recoil control and muzzle jump. Additionally, they can help you to maintain a more consistent grip on the gun, which is helpful when you’re trying to shoot accurately and quickly. So if you’re looking to improve your shotgun shooting performance, using a pistol grip can be beneficial.