I've definitely checked bags in more than 3 hours before a flight at EDI. During the snow disruptions last year, when there was havoc on the roads, a colleague took hours to get to the airport the day before I was due to leave.
So, I left *really* early, the roads were clear and I got there in plenty of time. My flight was EDI-LHR-LAX and I there was 2 EDI-LHR flights to depart before mine did (I was meeting a colleague there so didn't ask to get on the earlier flight).
The checkin agent had to phone someone, but they accepted my bag. I had checked in online and printed my own boarding pass.
I thought this was a Silver/Gold benefit - being able to drop bags earlier, so that you could spend time in the lounge.