I don't know about CS3, but I found this on another forum:
"In CS4, the TWAIN plug-in is not installed by default anymore because of the many problem it can cause with poorly written scanner drivers. If you don't mind running the risks of constant crashes, you can install the TWAIN plug-in from the Optional plug-ins folder, but you do so at your own risk.
Scanning outside of Photoshop, either with the scanner's stand-alone software or a utility like VueScan or $ilverfa$t is smarter and more efficient. There's very little point in tying up an expensive piece of software like Photoshop to do such grunt work as scanning."