The key to Warrior 3 is not in your hip, or your knee; the pose begins with the placement of the standing foot. If you allow the inside arch of your foot to collapse, you will instinctively bend your knee and use various parts of your body to compensate for the lack of balance. The knee joint will no longer be stable and that creates a chain of events within your body.
Before you start Warrior 3, come onto the outside of your foot. Use all your toes and grip into the ground. Now straighten the standing leg. Use the muscles just above the knee to straighten the leg. This is the foundation to each and every standing pose. Very much like a house, if the foundation is compromised, so is the rest of the structure.
You must not be afraid to straighten the standing leg. You have muscles around the knee joint that are designed to stabilise that particular joint. Focus on using muscles to straighten the leg and you will slowly lose your fear and start to feel more balanced, physically and emotionally.