Hi Bliss_0 - The language reference does exist, but its wrapped up in the Community Help. In Flash Builder, select Help > Flash Builder Help. The information that appears includes the ActionScript 3.0 Reference.
Note that both Flex 3 and Flex 4 use ActionScript 3.0. In other words, you will NOT find an "ActionScript 4.0 Reference."
If you prefer to review your help information from a web browser, look in the upper right corner of the Adobe Community Help window. You will see the URL for the page you are viewing.