So what is the difference? Mobility: the ability of a joint to move ACTIVELY through a range of motion
Strength: how much force a muscle can produce
Flexibility: the ability of a muscle or muscle groups to lengthen PASSIVELY through a range of motion While being flexible is impressive, having good mobility and strength proves to be more beneficial when it comes to your ADL's (activities of daily living).
A good test you can do on your own is this:
Lay on your back and lift your leg straight up into the air as high as you can before your knee begins to bend. This exercise is called a straight leg raise. Doing this will show you your MOBILITY. Now, with your leg straight in the air, place your hands behind your knees or on your calf muscle and see if you can bring your leg back anymore. This assisted movement will show you your FLEXIBILITY. Your muscles need to have good STRENGTH in order to lower your leg back down to the starting position with control.