What airline? A lot of them won’t let you checkin that early. Especially international carriers that only have one or two flights a day. For example Icelandair only opens their checkin counter 3 hours before their one flight a day and British only opens theirs 3 hours before their first flight in the evening.

Even if they are open, some won’t let you checkin that early. I flew from JFK to Madrid on Friday on Iberia and they weren’t going to let me checkin for their second flight until the previous one was done with checkin. Fortunately I have very good OneWorld status and only had carry on so the lady at the desk did it.