Some users have recently had their accounts hijacked. It seems that the now defunct EVGA forums might have compromised your password there and seems many are using the same PW here. We would suggest you UPDATE YOUR PASSWORD and TURN ON 2FA for your account here to further secure it. None of the compromised accounts had 2FA turned on.
Once you have enabled 2FA, your account will be updated soon to show a badge, letting other members know that you use 2FA to protect your account. This should be beneficial for everyone that uses FSFT.
from my own personal experience, not much. CS3 added the Spry Assets and a bunch of pre-made templates, that's about it really. Oh yeah, and a new splash screen.
The Spry stuff is just a bunch of pre-built JS code for doing AJAX type stuff, but you can easily find free code to do the same thing with a simple Google search. It also has nothing on the more popular JS frameworks out there now (jQuery, Prototype, etc...).
The templates would be useful to someone new to CSS layouts, but most pro's already have their own templates or know how to roll their own layouts.
All in all, I'd say if you already have DW8 and know what you're doing, then CS3 is a huge waste of money for very little improvement. However, if you have a really old copy of DW (like Dreamweaver MX) and want to upgrade, or this is your first time purchasing DW, then it would be worth it.
EDIT: there were reports of compatibility issues with DW8 and Windows Vista, but I believe those were addressed in a recent patch.