, depends. I have checked in for a flight over 3 hours before. The girls were at the counter, so I went up, and they checked me in. I had lounge access, so I could use that time to relax.
But you are right, usually they open 3 hours before, if they don't have flights before yours.
I would get there at 9am at the latest. If the specific airline has multiple flights leaving within 2 hours of each other, then it can get busy at check in. The line up can go from 3 people to 30 in 15 minutes.