All baggage queries answered here, within the site
http://www.baa.com/
If the check-in is quoted as 3 hours why be earlier? If you get there 4 hours before T/O you might not even find the check-in open - depends who you are flying with. Some have 2 desks for specific flights and others have "All BA flights check in here" with about 8 desks open. It is only more time to get bored or be ripped off in the shops!