This is one of my favorite drills to increase ankle dorsiflexion. It's inherently extremely simple and can be done anytime anywhere. One of the keys to increasing mobility is to spend as much time as possible in end range. Meaning performing mobilizations like these as much as possible, and then following them up with any movements that USE that ankle DF (think goblet squats, forward step downs, deficient heel raises).
By all means, you do NOT have to mobilize in exactly 3 directions, you can choose any number of directions you want. The goal of the "3D" is to get you thinking outside the box. Pure ankle DF almost never happens when we move, and our ankles need to be able to move into DF with varying degrees of calcaneal inversion/eversion.
Something I stress with patients who lack ankle dorsiflexion is proper gait mechanics. The brain/body is smart, if you lack ankle dorsiflexion at the talocrural joint, you'll more than likely maintain forward progress at the ankle through other joints - namely the subtalar joint and compensate with excessive pronation. Thus, if working on gaining true ankle dorsiflexion, make sure to USE IT when you ambulate! EVERY STEP IS A CHANGE TO MOBILIZE YOUR ANKLE!!