Here is a list of 3 letter domain name extensions available for registration on Micro Domains.
The most popular TLD in the world is the .com domain with nearly 50% of all websites having this extension. The next most popular domain name is .org with just under a 5% share.
A domain name with a 3 letter extension allows you to add 1 or 2 letters to the left of the "dot" and still have a short domain name of less than 5 letters long.
It is also possible to have a domain name that is just 3 letters long (not including the dot). Find a 3 letter domain name here.