I am interested in finding a way to facilitate constant 6-wheel drive utilising a centre differential that splits the torque from the gearbox between the 3 axles while allowing them to vary in rotational speed from each other.
Drive each wheel with a hydraulic motor.
Each can then be independently be controlled to match conditions at that wheel ie slip etc
To have a really good mechanical system will involve LSD's as well. Another configuration is an H rear axle with a chain drive to each pair of wheels.