Cesar Barscevicius has put this nice video, demonstrating a rack and pinion drive, with each pair of wheels (front and back) driven by one motor+differential and the third motor rotate the front wheels using a well-built rack and pinion. I could not figure out the power transmission mechanism in the front wheels, but maybe the builder will send some more details (and I’ll be happy for building instructions for this project!).

UPDATE: I’ve got an email for Cesar. Indeed he used 4 double bevel gears, inspired by 222doc “AWD buggy 4×4x4″ NXTLog project. He has an NXTLog project page for this car here. I asked him for more photos and maybe building instructions - stay tuned!