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The Flex 4 SDK is not supported in Flex Builder 3 design view. I'd encourage you to go ahead and download the Flash Builder 4 beta 2. This should be running fine side by side with your FB 3 (just make sure you don't convert you existing workspaces, but create new ones for FB4).
Here's the link http://labs.adobe.com/technologies/flashbuilder4/
I tried Flex 3 since we have only Flex 3 licence.
I read somewhere that if we install Flash Player 10 and make changes in project setting to point to flash player 10.0.0, it wioll work.
I tried that too and it didnt work.
Will push for Flash builder 4 licence if I find it comfortable with the beta.
The link that I posted in my previous reply is for the Flash Builder beta 2 - the beta 2 is free. You can download it and play with it, and you don't need to pay for a license for the beta version.