The following video (below) gives us a quick example of what the three-hop test is. In a nutshell, our athlete is taking three consecutive hops using the same leg while striving for the maximum forward distance possible.
Note that the participant must maintain total control of the hops: no loss of balance, stutter stepping, or touching the other foot to the ground is allowed. I will typically allow the participant to practice the skill a few times to become acclimated with the movements.
Once the participant is comfortable, I will measure the distance of three successful trials and calculate the average. This average can then be compared to a re-test at a future time, or it can be compared to the results of the other leg.
In rehabilitation literature, this test has been used for measuring strength and stability outcomes of knee surgeries (such as an ACL repair).
I also personally use this test in non-surgical patients to determine if there is a significant difference in leg strengths or preferences. I will at times find a huge difference in the distances between the right and the left leg. This can indicate a problem with a potentially weak or under-utilized leg.