Watch The3Tails: A Mermaid Adventure for Free: How to Stream the Movie Online at 123Movies. . Emily, Selena and Jackie - known to their 50 thousand YouTube fans as The 3 Tails - finally bring their journey to the big screen. Their mermaid adventures have enchanted fans for more than five years, as the girls struggle to understand the meaning of their special gift, protecting their secret from the world. Even their parents don't know that they have the ability to transform into mermaids. In The 3 Tails: A Mermaid Adventure, their lives are threatened after a stranger learns of their abilities and kidnaps one of the girls in order to harness her DNA. During a perilous ocean rescue mission, they discover that mermaids are not the only endangered species in the sea.