It can fill all 3 roles, easily, simply by swapping out barrels.
Role 1--Home Defense.
Put an 18" barrel, a +2 magazine extension (will replace standard barrel nut) & mag spring, and if more onboard ammo is desired, a good stock-mounted, velcro/elastic shell holder. Perhaps mount a light on the mag extension, if desired. Put a tritium front bead on--for close-in work, you don't need rifle or ghost ring sights; a luminous bead is better (imho--ymmv). (FWIW--using Foster-type or rifled slugs, this set up will take down pretty much anything in North America within 100 or so yards, depending on your skill level/comfort, so could easily fill two roles...)
Role 2--Clay Birds & Meat Birds.
Put a 24-30" barrel on (depending on personal preference & type of hunting--clays, waterfowl, upland birds...personally, I've found that a 28" is a great all-around barrel). Put the original magazine spring/follower in (and the limiter, depending on your state laws), the original barrel nut, and you're ready to go. Will also work great on small game.
Role 3--Big critter slayer.
Get a 24" rifled barrel with a cantilevered scope mount. Get good sabots. Drop deer out to 200+ yards with aplomb.
There you go...how an 870 can fit all of your needs.
No need to change the furniture around; the standard stock set will work just fine in all three roles.