I don't get this joke. Do you?

One time, my older brother and my Uncle were telling jokes. My Uncle said a joke that went something like this. *One time there were three ducks and a waterslide. The first duck goes down and says "Wee!" the second duck goes down and says "Wee!" The third duck goes down and says "Radio!"* I am assuming the joke was sexual somehow, but I don't know. Perhaps I missed something in health class? Or were they just screwing with me and the joke means nothing?