No... serious photographers DON'T need CS3.
It's not the software you use that makes you a photographer, just as its not the camera you use that does either. Put either into the hand of someone not "serious" about photography, and they'll turn out sludge. On the other hand, put either into the hands of someone who is skilled, and taken the time to learn them, the results can be astounding.
My personal feeling is that I'm not being limited by PS Elements, so I don't see the reason to upgrade to CS3... even though the tools may be nice to have! I say look into the features to determine if it is worth it to you at the present time. $299 is much cheaper than sticker price, so I can see how it would be tempting too.
(I have LR (bought early at discount) which fills the area where Elements was lacking for me... RAW conversion.)