I have a 20 gauge 870 express shotgun, it says on the receiver "870 express magnum" and I'm assuming that really is a magnum. I've found a few places online that say there should be a M at the end of the serial number but mine has a U. Just would like to know if I can shoot 3'' shells out of it (the barrel says the barrel will shoot 3s if the receiver is a magnum).

Yes you can absolutely shoot 3" magnums out of this shotgun.

Serial number suffix letters tell me exactly what you have.
"U" designates your gun as a 20 gauge Light Weight (LW) Magnum

The "M" that some are telling you it should have was/is only used on 12 gauge 3" magnums and never on 20 gauges.
Many decades ago Remington built the first 20 gauges on 12 gauge frames and the 3" magnum 20 gauge version of this "heavy frame" shotgun will have a "N" suffix in the serial number.

Awesome thanks so much!

Welcome to the Remington Owners Forum tucker21 !!