If you've got LR set to send the photo to CS3 as a TIFF, it will create a new one off your original every time. LR will not let you directly edit an original file. Now, once you have sent the photo to CS3 and done some editing there, you can save back to the file that LR creates. And, if you haven't already found it, you can set the format of the file for external editing to either TIFF or PSD. Right now it appears you have it set to TIFF. If you don't want that, just go to Edit -> Preferences and then the External Editing tab and set it there.