As of this morning-Dec. 15th at 10:00am Central, the main pages for Photoshop CS3 on Labs.Adobe.Com are not yet up. There are some links that have been floating around that will allow you to actually download the installer, but so far the page for obtaining a CS3 beta is not yet working according to John Nack.
From a post on his blog: “I awoke to discover that people have been poking around Adobe servers and are now complaining that the site isn’t accepting their CS2 serial numbers. There’s a reason for that: the site isn’t ready to go just yet, and we’ll post an announcement when it is. So, while the interest in downloading the new beta is encouraging, I hope people can chill for a little while.”
So, hang in there just a bit longer…the main https://labs.adobe.com/technologies/photoshopcs3/ should be working sometime today.