What irritates me the most in Flex Builder 3 (windows) or Flex3 for Eclipse (linux), is when you upgrade SDK, you loose autocomplete or some packages cannot be found by autocomplete.
For example, most common error is when you write import flash. and want to get events, all you can see is flash.errors but not flash.events or any other.
Problem: (this is just pure speculation)
Since FB3 comes with FlashPlayer 9.x, and most probably that you have 10.x by now, adding SDK looses itself between versions. No matter if you set in SDK’s ini file that target player is 10.
How to fix it:
Go to your project properties -> Flex Builder Path, choose Library tab, and expand your SDK.
DELETE playerglobal.swc then add playerglobal.swc manually from SDK.
(same-same but its somehow very different).
Now, clean your project and WOA! Now you have autocompletion and can see all packages.