All LLL .com's, .net's, .org's, etc are already registered, so you can only make offers to their current owners and see if they accept it.
Depends on which extension you're looking for? Because there might be possibility to get 3 letters domain name in few extension that are generally overlooked.
Very difficult to get it. Better search the domain in any registrar and view the profile, if for sale then buy it. This waht we done last time to buy our domain