You guys are all making a really good case for a Part 5 of this series - how to apply the Red/Black game to dating situations, and how girls will behave towards you with different things on the table.
Do you all want to see this? Let me know and I will write it.
To the points above - yes, what you are describing is cockblocking, which is two girls playing Black with each other and Red on a guy, in order to prevent one of them from going home with you (which to them is seen as their friend playing Red by choosing the guy, so the girl who will become the cockblocker would lose - and be alone - before she cockblocks). Once she cockblocks....no one of the three is getting laid, but at least the girls have their friendship intact. Remember that sex is short term (the guy could disappear forever the next day) but friendship/social ramifications can last a lifetime. So even the girl being cockblocked is (somewhat) actually okay with this, at least publicly (both girls are probably mad as hell inside, though).
How does one avoid this? You've gotta play Black with the cockblocker and "give her something" so that she doesn't feel shafted. Most often that takes the form of another guy, some kind of validation, or a threesome.