I know there's decent money to be made, but I've never gone down the 3-letter domain route, so I'm pretty focused on "words".
For a new company, they have 4 basic choices (perhaps more?)
A) Use a "brandable but real" word, and give it a different meaning through marketing and advertising. For example, "Amazon". An extension of this is to put a couple of real words together (Red Oblong) and do likewise
B) Use a descriptive generic name (hard to trademark/defend but "it does what it says on the tin") - and it's easy to know what the matching domain should be!
C) Use a 3-letter acronym (or for the deeper pocketed, a 4-letter acronym... after all, you're going to need to do a LOT of marketing)
D) Use a completely made up word (eBay, Woot, Froogle etc.)
A) is going to be easier to advertise than C) or D) in many cases. B) will be easiest, but won't suit those with 7-to-9 figure marketing budgets as they're not "creating a brand"
Note that if you NEVER intend to go down the "branding" route but are simply using the name as an "umbrella" so that when you talk about your company, that is what you tell people as you hand over your namecard or marketing materials, then it doesn't really matter what you use since you don't particularly care if people remember the company name or not.