This article in The New York Times sheds some light on the dynamic of these three girls, stating things that I have not read in other articles. Just like we speculated three may have been a bad number once again. One (the birthday girl) which is MG
was very close to the victim. It is stated that AW and the victim were
not close. As we know, MG did the actual stabbing and and although AW introduced MG to Slenderman, MG was the one that had the idea to kill so that they could become proxies.
Very interesting.
The "third girl" is AW and there is another paragraph about the dynamic.
http://www.nytimes.com/2014/06/08/u...s-to-cope-with-slender-man-stabbing.html?_r=0