Crutches can be used as single or a pair.  Using crutches can help keep weight temporary off from the injured leg , assist with balance and enable to perform daily activities safety.  It requires upper-body strength thus active individual who suffer with short term one side lower limb injury would need it during healing or recovering period. Not ideal for long term use. 

Walking cane provide a third point of contact with ground that improves balance. Now it modified with wider bases that enhance stability. Individual who is struggle with one leg weakness and require one hand support to keep balance for smooth walk.

Walker is the most stable mobility aids because they have four legs. Individual who is struggle with balancing due to both leg weakness / back issue and require two hand support for safe walk. It can be come with or without wheels, depending on what is more comfortable for the user.