I own a few domains, all .com, but that is all because the major market for those designs were in the USA.
As for the domain limit, yes its three characters minimum, except when a submission is made to the domain registrar board, you can obtain two letter domains from them if there is a good reason. e.g. www.bt.com, www.3m.com, www.bp.com....
One letter domains are not available due to thier limited scope, i.e. there are not that many that can be registered!
m@