Mermaids from the coasts of Scotland. They are attracted to human males and their offspring are known to be great sailors. When captured they will grant three wishes, after the wishes are granted she will disappear

I swear I've heard this legend many times (the movie Aquamarine comes to mine). Obviously, they're wish-granters and all, but how do you bind a spirit meant to disappear after 3 wishes? And do they act like other mermaids?