In my view, there are better investments than 2 and 3 letter
acronyms in the domain name sphere...
Click over to DNJournal's weekly sale list...
http://www.dnjournal.com/domainsales.htm
Note most of the highest priced sellers are NOT acronyms, but rather one, and often two word, dictionary domains.
Shorter is not always better when it comes to value. A noteable exception to that are short domains that are actual dictionary words, such as "ax", "bed", "sex" ...
To be clear, short acronym domains have some value for sure, but much of it is hype ...
Ron