Problem of installation xcode4 and then xcode3 I install Xcode4 first in the default path /Developer then I install Xcode3 in /XCode3 It seems that the installation was not completed because I can't see Xcode.app in /XCode3/Application. What should I do?

Update!
I noticed that while I'm installing Xcode3 in the window "Install Xcode and iOS SDK" in path of Custom Install the check box of Xcode Toolset under Essentials is disable with unchecked status. What should I do?
I think the way to have Xcode 3 and 3 is to install Xcode 3 first - it will be in /Developer. Then Xcode 4 install will move Xcode 3 to /Developer-old  and install Xcode 4 in /Developer. So in your case delete /Developer and start again.