A 2 3/4" barrel will fit onto a magnum receiver without a problem and cycle properly. However, the other way around, a 3" capable barrel will also fit onto a non-magnum receiver, but will not eject the longer spent shells as the 2 3/4" ejector does not accommodate the longer shells.
On a different note, as far as I know, Remington never made a factory non-magnum 18" barrel.
If it's a true 18" barrel, it would be 3" capable, thus a magnum barrel.
If your barrel is indeed 18" long and only accommodates 2 3/4" shells, then according to our laws, you have in your possession a barrel that has been cut down below the legal size of 18 1/2" and as such created a prohibited barrel.