I would choose Trunk Bay first, Pinel Island second and turtle cove third. I have been to all three. Trunk bay is the most beautiful. You anchor and have to swim to shore to spend time on the beach, but the coral reef is amazing and you can easily spend most of your time snorkeling. It is not a commercial area with food or anything. On Pinel Island, you can eat and rent jet ski's last time I was there. Turtle cove just has turtles, not many fish. Email if you have any other questions.
...great beach, for sure, as is Magen's on St. Thomas. However snorkeling is better at Maho, Honeymoon caneel?, Cinnamon, etc. None the less, Trunk does have pretty good snorkeling, but I would take the other sites if snorkelling was my primary motivation