Hey, first time hunting with a shotgun. I have a 12 ga Stevens 320 18.5" smoothbore barrel. What will 3" HP slugs do to the deer? Am I better off sticking with the 2.75" slugs? I usually use a rifle, but to hunt where I live, I needed a shotgun.

I honestly don't think it is NEEDED, you'll have a heavier slug, more power, BUT you'll have more recoil and more cost.
I use a 2 3/4" because of the reasons listed above and it patterns great in mine (2" groups at 50 yards).
I would think they'll do about the same damage wise but with more lead to pick out...
buy a few boxes of both and see which shoots best in your gun.
Just my $.02!

I have to agree w/ Toms
have shot a few with a shotgun
have used 2 3/4's 3" & 3 1/2's you will get more power & range moving up
but I found for me I flinched a lot more & didn't hold a good group with the 3 & 31/2's so I stick with the 2 3/4's they will put a deer down just as good without the extra punishment to you

here is the deal. And yes I did the research. 2.75 is the best most of the time. when you move up to 3"-3.5" shells all you get is more shot or a bigger slug.As for the powder is all the same in all 3. so you have more lead to throw with the same amount of powder. I would say for bird hunting this would be fine,but with buckshot or sligs 2.75 is the way to go. Best thing to do is get a rifled slug barrel and shoot harnady rifled slugs. they or awsome

2 3/4" slugs are the way to go.

I will be using 12 ga 3" Remington copper solids in my scoped Rem 870 in a couple of weeks for Ohio gun season. I hope to see what the performance is with these. When I sighted this rig in it shot great groups beyond my expectations. What's the big difference with the Hornady shell?

A 2 3/4" slug is plenty, and will blow massive holes threw game like deer.
If u hit them well