I am trying to find a book that I read in my teens (mid 80's). I'm guessing it would have been published in the 70's or early 80's.
It was about 3 kids (teen boy, teen girl, teen girl's younger sister) who end up lost/stranded on an island for several weeks. I thought the title was something like "Three on an Island", but a search of that title wasn't fruitful, so it's possible that I am wrong about the title. Thanks for any help you can give me. 
