IMO I would look at buying a new 870 in a 3" mag. By the time you convert your existing 2-3/4" 870 over to 3" mag, you will have 1/2 to 2/3 the cost of a new 3" 870 invested. Not only are your looking a having the ejectors repositioned and re-revivited, maybe reblued, but then you are looking at either having your existing barrel rechambered (existing chamber deepened) and maybe the forcing cone re-cut and honed. It maybe better just to replace that barrel with a new 3" barrel. I would say a ball park figure for the complete job, for quality work from a good gunsmith, would run about $300-$350, without cutting any corners.