Can a 2-3/4" 870 Wingmnaster be made into 3" by only changing barrel & ejector

[Banshee]

Yes a 2-3/4 can be converted to 3 inch by changing the ejector and barrel. The ejection port is the same length on both so nothing needs to be modified there.
Yes sir, it takes a special tool to cut the heads of the rivets off of the ejector. Brownells sells this tool. Can you do the conversion, that depends on your ability and equipment you have access to . It isn't hard to do, but you can make a mess out of it if you aren't carful.It isn't something you want to tackle with just a hand drill.

Do you have a need for 3.00 inch shells? (Not being an ass, just curious.)

[Synchronizor]

One small correction: since your 870 is a 12ga, it has both an ejector, and an ejector spring. Both of those have to be changed out for 3" versions in order to properly eject the longer hulls. But the spring fits into the ejector and is held in by the same rivets, so it doesn't really add that much to the process.

I have heard of some people removing & replacing the ejector & ejector spring with more basic tools like pliers and normal punches. That can work, but you do need to know what you're doing, and the end result may not be as good.