I've seen short urls like that used by e.g. shorturl.com who give you an extension like yourname.3nd.com in which case if it sounds/looks reasonably professional then you could use it or market it in that way.
Similarly if it looks like it stands for something then its usually a good bet.
Or if it has a l or i or p at the end it can sometimes relate to a limited, inc, or plc etc company eg o2p might mean 02 plc
If it's a memorable set of 3 that sticks in the mind then someone might buy it for their email address to say to people when they're out and about - easier to remember than long stuff, easier to txt to people, easier to write on back of hand etc etc
But as has been mentioned it depends greatly on the actual domain.
Don't pay too much!
(my eternal problem!)
-aqls-