I have the characters in 2D, already rig in afterEffects, but the client wants to change the characters from 2D to 3D. What software would you recommend me to draw 3D characters? Also, any advice would be useful to me, like the advantages / disadvantages of 2D / 3D animation. Any plugin useful for AfterEffects to animate 3D characters?
If you have no real budget take a look at http://blender.org. It's free, open-source, incredibly powerful, fairly easy to integrate with After Effects, has a great render engine, and there is a very good user support group and set of tutorials right on the site. I use Blender to do a lot of designing and some rigging, but I don't really do Character animation.
If you have a decent budget then I suggest Cinema 4D. It's powerful, has some great features, is probably the easiest 3D software to integrate with After Effects, and there are a lot of really good tutorials right there on the Maxon site to get you started.
If you are completely new to 3D expect to spend about a week just figuring out the UI and how to move animate things. You can then easily spend another week learning about texturing and lighting. 3D animation is not quick, not easy, and will probably never be "push the button and magic happens." You will have to utilize the pencil test/ink and paint workflow. That means, rough out the shapes, rig them, make them move, then fine-tune the shapes and the blocking and the camera moves. When everything looks good your pencil test is done. It's time to do Ink and Paint, which is texturing, lighting, and all the rest that goes with completing a shot.
I concur with Rick. You can't expect to flip a switch and just move on to 3D. A lot of stuff works completely different, be that just the extra transform dimension that needs to be considered, especially in your case for rotations on a character rig's joints. This will take time to learn, even more so since CA is considered one of the crown disciplines of animation and of course designing the characters in 3D in the first place is a completely different matter. You may be completely in over your head on this one and unless you really have a year time to learn all this stuff, I can't see a realistic way you would be able to do it. And there are of course budgetary considerations as well. Time to talk to your client and find collaborators experienced in 3D...