1) Graphics: In terms of graphical quality, Driver 3 is way better than Driver 2. The first one had this Doom-style, wich is out-dated even for GBA. Also, the game was pretty slow, and the fps was low. Cars were tiny and the 3rd person view of Tanner looked like it was being generated directly from your Atari 2600. Driver 3 has some of the best textures on 3D objects GBA can support. The textures are clean, colorful and crispy, and the fps is better than NFSU2, wich, in my opinion, has the best 3D graphics on GBA(The last one is not so good). Tanner now looks like a rendered 32-bit generation character. The only real problem with graphics is that the nearest polygon lines from player's perspective have this distortion-movement like. It happens when the polygon line "brakes" and start looking almost like a curve. This is the same problems that used to happen with some first generation 32-bit games, mostly on early Sega Saturn games.
2) Now let's analyse playability. In Driver 2, playability was what turned it into one of the best GBA games in 2002, 'cause it played so much like the Playstation version. Overall, driving experience were very precise, and very soon player's would be able to avoid traffic and walls, using handbrake in a very precise way(the response and physics were really good). Driver 3 has no resemblance to the first one on this subject. The car seems to float over the road, and the strangiest thing is that you'll drive better you car if you use ONLY the handbrake WITHOUT letting go the accelerator button. But, once you realize this, it won't be enough to stop having some undesired collisions with other cars or walls.
3) In terms of fun you can get, I would say this is up to you. My opinion is that the first game is better. Regarding a bunch of repetitive missions, altough fun enough to keep you playing it for a time, I say both games are similar. Driver 3 has a little more variety, as player's are able to fight and drive boats(altough boat driving experience in this game is no different at all that car driving). Unfortately, there are no car damaging at all and police doesn't chase you. This is the factor of wich makes Driver 2 better than this last one.