I would recommend not cutting the tendon, while it is common practice that does not make it right, and sets up for problems later on. The muscles and tendons work together, your disrupting the system of checks and balances by severing the connection. Locking stifles seems common in young horses. When my QH was right around 3, 15 years ago, he had the same problem. My vet told me to strengthen the muscles in the hind end by doing lots of long trotting going straight, back up hills, climb hills and the problem took care of itself. It took about a year for the problem to permanently go away but I was in no rush. In my opinion take the time to let her muscles develop and see if she grows out of it.