I know check in is at 3 but we did the online checkin- we can skip the front desk but what time does your room number pop in the app?
Whenever it is assigned - people often get them before they’ve even got on the plane. But though check in is at 3, they don’t always have a room ready for you at that time. Could go either way.
As @missoverexcited pointed out, when it is assigned. It’s really no different than the front desk check in except that if you go to the front desk you can ask then to try and find you a different or earlier room if you made a specific room request when booking (upper floor, lower floor, etc.)