how to get a full version of Adobe Photoshop CS3

Once your trial period has expired, you won't be able to reinstall it to continue your trial because, even if you uninstall it, it will leave a record to show that it was previously installed. Downloading from a different site won't help because the installation package will be the same (if it's legitimate). The only legal way of continuing to use the software is to purchase it and thereby get a serial number. You might consider the very good free alternative, Gimp. This has practically all of the functionality of Photoshop (certainly everything that the very large majority of people ever use) and is issued under the GNU Open Source license. You can read about it and download it from the official site here: http://www.gimp.org/features/ or from many other sites which offer tips, tutorials, forums &c. Here is just one example: http://www.getgimp.com/features.php