This^^^Check In isn't the same as getting into your room. Early AM they should be able to check in, but odds of getting your room much earlier than 10 or 11 is pretty slim. Check In time is 3 (or 4 I can't recall) and they need to turn the room over from the previous guest. There is a chance that you would be assigned to a room that was empty the previous night, but WDW has pretty high occupancy levels so most rooms are occupied.
I always understood that the roll over time for the new day was just before 6am. At any rate, we often check in between 7-7:30. A few times our rooms have been ready for us. The rest of the time we leave our bags with luggage services and head to a park. If you arrive in the wee hours of the morning, you would have needed a reservation for the previous night in order to get into your room.
We have checked in several times around 7:00 am arrival day. The latest our room was ready was noon. I always note that we get there early.
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