“Two of them must recuse themselves.”
The SCOTUS has no rules or requirements for the recusal of Justices.
http://www.nytimes.com/2011/12/01/opinion/the-supreme-courts-recusal-problem.html?_r=0
http://articles.latimes.com/2012/feb/12/opinion/la-oe-segall-kagan-recusal-20120212
Recusal is a function of natural justice?