Older Remington 870s were mostly set up for 2 3/4" shells only. More newer guns (including Express guns) are built on Magnum receivers, and those can be determined easily- the serial numbers of 12 ga. 870s with magnum (3") receivers end in an M. Standard receiver (2 3/4") 12 ga. 870s have serial numbers ending in V.
You can put barrels with 2.75" chambers on Magnum receivers, but it isn't really a good idea to shoot 3" shells in them since the chamber isn't long enough to accomodate the mouth of the shell as it uncrimps. You can put barrels with 3" chambers on standard receivers, but the receiver isn't set up to handle 3" shells- the ejector is mounted too far forward.
You really don't need to do any more than a 2.75" shell will do for home defense, and unless you are a serious waterfowler or turkey hunter or you need an Alaskan bear gun, the shorter shells will handle better than 99% of your needs.
If you have any doubts about how a barrel you are considering for purchase retains the magazine cap, ask the seller...
hth,
lpl/nc