its not compulsory to check in 3 hours before the flight but check in will be open at least 3 hours before departure. latest check in tends to be approx 90 mins before departure. the reason why, your flight could have 200-250 people on board. the check in agents neeed time to check everyone in and get the load sheets prepared so the flight can meet its slot.
i fly to ibiza on monday at 730 and much as i loathe to get out of bed earlier than i absolutely must i will be at the airport by 430 simply to avoid all the queues.