If you want a more legalistic opinion, you can post on the Legal Forum here at DNForum. However, I have enough experience that I can give you a good answer.
A big company can always decide to come after you and try to take a name. So, no matter what you do, there are no guarantees on how this hypothetical company will react. They may have an attorney on retainer so that the effective cost for them to write you a Cease & Desist letter may be $0. They could also go beyond writing letters.
On the other hand, a 2 or even a 3 letter domain is pretty generic, inherently. However, if the company is usually known by the domain or if they have a trademark, especially a registered trademark on the 2 or 3 letters, then you will probably lose the name before a WIPO panel. Beyond losing the name, you are unlikely to have any additional penalty. For example, if you grabbed GE.com or IBM.com, forget it. It would be highly unlikely that those companies could lose those kinds of names. They would threaten you with the $100,000 penalty for cybersquatting, and you would quickly surrender.
If the company is say United Airlines, and they are known primarily by unitedairlines.com and ual.com, you might be able to hold onto ua.com. Then there is United Artists, so you could have 2 companies after you.
If the company is Abraham Cars, they are much less likely to come after you. Even so, there could be other companies with the name AC.com who could try to take the name from you.
Good luck.
