My three-year-old Quarter Horse/Thoroughbred filly is a little shaky in her front legs, mostly if excited or after she has been worked. It doesn't seem to bother her as much as it concerns me. She may be slightly over at the knee but not much. Is it because she's still young? Will she outgrow it? I don't work her very hard; I just started riding her.
Your filly is likely a little shaky in her front legs due to her being over at the knee. With fatigue or excitement (as you have seen), the shakiness becomes more obvious. We assume that her feet are normal and that her fetlock angle is normal. Since she is three years old, it is unlikely to improve. However, if she is somewhat club-footed and very upright in the fetlock as well, you should have a veterinarian knowledgeable about feet and legs examine her. Some surgical procedures can help, but the older the horse is, the less responsive to surgical intervention it may be.