In Bridge CS4, go to Edit>Preferences>File Type Associations and change the association for PSD, TIFF, JPG to Photoshop CS3. Check that the same has been done in your OS file associations (don't know where this is on a Mac). Then whenever you open one of those kind of files through Bridge, it will trigger Photoshop CS3. The other thing you can do is open Photoshop CS3 before you open any images. Once CS3 is open, any images you open will open in CS3 (at least it works that way in Windows - you can try likewise on your Mac). But alas, if you want to keep Bridge CS4, you will have to keep Photoshop CS4.