This takes some explanation.
Yes, you can fire 3" shells in a 3" chambered barrel on a 2 3/4" receiver, BUT the gun can't eject the longer shells since the 2 3/4" receiver doesn't have the longer 3" ejector assembly.
The only difference between a 2 3/4" receiver Remington and a 3" Magnum receiver is the ejector assembly, and the Magnum markings on factory guns.
The length and heat treating is exactly the same on both.
You can have your receiver converted to a 3" receiver by having a gunsmith with the special riveting tool install a longer 3" Magnum ejector assemble.
Among others, Wilson/Scattergun does this for a reasonable fee.