It sounds as if they were dabbling with Enochian calls, trying to summon fallen angels to do their warped bidding. So, I'd think any symbol tattooed upon the three girls, if not actually an inverted three, would be either Hebrew or so-called Enochian. There is no different symbol in Hebrew to fit the description, but the Enochian "Tal," corresponding to "m," does. Not sure what to make of that ... Moloch?