There are 17576 different combinations of three letters (or 46656 if you include numbers), so the question you've got to be asking yourself is will demand outstrip supply. Personally, my hunch is that it won't and in the next few years there will likely be another TLD or two become available and anyone who really wants a three character name regardless of the TLD will have that option. Of course though if you manage to get a combination of three letters that is really valuable (take SEO for example), then yes it may well be worth something.
what do you think about four letter .com domains that are not containing letters such h, j, q, u, v, w, x, y, z ? have they some worth ? thank you
I dont think that just because a domain is 3 letters it must be worth a lot of money, I think this is just a trend. For example http://xzkw.com/ is a real web site, would you be prepared to pay hard cash for it, I dont think even Polacs can pronunce this thing. Now another example http://cheapwebhostinghouse.com/ I am sure that in a few months this weebsite will be first for cheap web hosting at least in MSN. And MSN controls 9% of searches! So which one as a potential to make money?