DW3 Extreme Legends added Mosou mode for the seven neutral characters, which was previously unavailable. Plus, every character now can retreive the hidden 5th weapon. There were also some other minor enhancements like the ability to give each bodyguard individual orders, change the bodyguard appearances, or the choice to choose an officer character as bodyguard. This will add some replay value to Dynasty Warriors 3, especially if you've already finished with all the characters.
But, keep in mind that Dynasty Warrior 4 will be coming out in the near future (March 25?). So you might want to consider that before you make your purchase.