A four wheel drive (4WD) usually uses 3 differentials! It has 2 sets of powered wheels (front and rear), and each pair needs a differential to split power on turns.

A third differential is used to divide power between the front pair and the rear pair (when the vehicle is sliding, keeping the wheels rolling at road speed helps prevent the tyres from skidding).

But if the vehicle is stuck with any wheel on a loose surface (e.g. if a wheel is spinning on sand), all power will go to that wheel, and you'll never get out! So this differential can be locked, to allow power to get to the other pair of wheels even while one pair is spinning.