Is there a reason to buy a DinA4 Wacom Intuos3 if I already have a Wacom Intuos2 DinA5 tablet?
The answer is: Are the additional features and size worth the money for you.
If I remember correctly the I3 has a differetn layout for it's buttons (it's symmetric so eaqually useful for lefties and righties) and there is the Touch Strip that can be useful if once you knowhow to use it.
For the full feature list you can try this link: http://www.wacom.com/productinfo/intuos.cfm
The pens and the tablet have better ergonomical features and other little improved bits.
And the 6D Art Pen works with the I3. More inofrmation here: http://www.wacom.com/productinfo/accessories.cfm
It's not an revolution but a looks like it's a nice evolution. The only question is: Is it worth your money?