A three way tripod head has separate adjustments for horizontal movement (pan), fore and aft movement (tilt) and camera orientation (vertical or horizontal). This makes a three way head heavier and more bulky than a ball head, but it’s easier to make precise, fine adjustments in one direction only.
Three way heads are popular amongst photographers who need to frame scenes and objects very precisely. They are closely related to two way video heads, where the videographer needs to pan and tilt the camera separately.