I replaced the factory OEM barrel on my Wingmaster with a factory Remington 19 inch barrel. My original barrel was stamped 2 3/4 inch, the new barrel is stamped for that as well as 3 inch. Using 3 inch magnum would be ok? Is the receiver different for 3” shells?

Your shotgun is a 2 3/4" chamber according to your original barrel, so that is all you should use......is 2 3/4" ammunition.
Yes, your new barrel will allow you to chamber 3" shells, however you have a 2 3/4" receiver.

Actually, you may have a 2 3/4" ejection port. This would prevent a fired shell from being tossed out of the receiver.
At one time the factory cut different ejection port sizes for standard and magnum models but eventually switched to the longer one as a cost saving measure. Barrels did not all switch to 3" at this time which is why I did not make a definitive answer.

The cheap way to determine if the port is the short or long style is to feed a fired (because the shell is measured by the fired length and not the unfired length) 3" shell into the barrel and see if it ejects. Try a couple different brands of possible as not all measure an actual 3" after firing.

If the empty does not eject, one can either resolve themselves to 2 3/4" shells or they can enlarge the ejection port to fit. I have seen this mentioned in the price lists at a couple of gunsmith's, Ahlman's in Morristown, MN being one. I haven't done this myself but I have seen it on a couple other brands including a Winchester M-12. If you go this route, don't worry about added wear and tear on the gun due to 3" shells. Pressures are the same between the two and the parts and reciever are the same except for ejection port.

The 870 has worn the 19 inch barrel for some 10 years, and the 2 3/4 inch shells are all I have used. I was digging around in the locker yesterday, and found two boxes of #4 Buck 3” shells. Made me curious. Thanks for the info, it is much as I thought.

I don't know anything about the size of the ejection port as I never encountered one that would not eject empty 3 inch shells. I have converted numerous 870"s to handle 3 inch shells, the ejector and ejector spring need replaced with the proper version. The 3 inch version has the stop is cut further back. Just note that this installation entails removing two rivets to remove the old and two new rivets to install the new. Brownells sells the rivet set to make the easier. After the rivets are filed down flush with the left side of the receiver it normally needs reblued.