How can I map a middle click to control click?
Try holding the Command button instead. A Command click is a middle click. Control clicks give you a right click.

What kind of Mac? If you're using a Magic Mouse you can use a program like MagicPrefs to turn a three finger click into a Command click. If you're using the Magic Trackpad or the built-in trackpad on a MacBook/MacBook Pro there is a program called MiddleClick I've used that adds a three finger tap/click ability to map to the Command click.