DD,
IMO your 3x9 should work just fine. If you ever change scopes,consider one with 50 objective. Will give you 15+- minutes addt'l shooting time at dawn/dusk, particularly in heavy cover.

I have found after 25+yrs of slug slinging 3" slugs don't do much but cause addt'l pain.
2-3/4 slugs have come a long way in 10-15 yrs. They will cleanly take deer out to 150yrds.

With slugs I like to start at about 30yrds to get a decent group and then back off to 50-75yrds.
Then mark off 125 yrds and try that. Then go back to 50yrds and shoot. I've also found that a good cleaning after 3-4 shots(with sabots)can make a big differance in groups.

Bottom line is - you might get lucky and only shoot 2 boxes of slugs or 5+ before you find out exactly what that lead slinger is doin out to your desired range.

Good Luck