Think about which market the 3 letters will belong to and whether its going to generate interest/income if you are unsure. If it costs a lot to reg then do some research into who/where you can sell the domain - who'd be interested in it and how much do u think they will afford to pay for it.
Ps. Since you don't have a domain name to appraise and this is more a general question i think you should have posted this elsewhere not in appraisals. This section is for domains you own already.