Something like "QZX" I wouldn't bother with, but if you're talking 3 letter words like hit, car, bat, day, red, sex, net, dog, cat, get, rob, yes, gif, ten paired in a CA, totally useable and nice. If they're something along the lines of RAC, FDA, LOE etc (slightly better on the usability end, for company abbreviations), I wouldn't be over the moon with them IMO. My confidence would be with the likes of a nice ORG, in terms of consistency, if going down that 3 letter road. That being said, I haven't sold or bought any CA domains in the past, can't really talk from experience. If someone has, their input may be more meaningful than mine