Yes sir, 3" chamber can load both 3" and 2¾" shells.
Specs on this shotgun
Simple, economical, functional, the 12 Gauge Stevens 320 Security is a capable home defense shotgun. Incorporating dual action bars and proven rotary bolt design this shotgun has an 18.5" barrel with ghost ring sights. Additionally the 320 utilizes a 3" chamber and fixed cylinder bore.
Specifications and Features:
Stevens Model 320 Security Pump Action Shotgun
12 Gauge
3" Chamber
5 Round Capacity
18.5" Barrel
Cylinder Choke
Ghost ring sights
Rotary Bolt Design
Dual Slide Bars and Side Ejection
Overall Length: 39"
Weighs 6.85 lbs

This gun is chambered in 3", so will take both 3" and 2 3/4" shells.

Mike, yes Sir. It has a 3 inch chamber and shoos both no issue. Cheers and stay safe!