As a Flex 3 developer, there are a number of links I think you should have. Some of these are tools that help me perform tasks faster and better, others are for quick reference and news.

Tools
Component Explorer
Style Explorer (CSS generator)
Color Picker

Plugins
TODO / FIXME
XML Buddy (XML color coding, validation…etc)
Subclipse (Subversion source control support for eclipse)
Perforce (Source control)
FlashTracer (FireFox Plugin)

Reference
Language Reference (Live Docs)
Runtime Error Codes
Compiler Error Codes

News and Community
RIA News Aggregate
Flex.org
Flex Developer Center
Flex Coders (Yahoo Group)

Blogs
Alistair McLeod
Christophe Coenraets
Ely Greenfield
RIApedia (Mike Potter)
Everything Flex (Rich Tretola)