Please help me to migrating Widget flex 2.5 to 3.0 in flash builder 4.6..
Here are some blogs you need to read (kind of old news):
Esri’s Roadmap for Web Developers | ArcGIS Blog
Final Release Support Plan for ArcGIS API / Viewer (Flex, Silverlight) | Support Services Blog
Bottom line is you should not be investing any time in Flex development.