How long will that planet be in a stable orbit? I am guessing not for long.
One can assume it's been there for a very long time already. Given the proximity of the planet to the one star and the fact that the other two are binaries of each other and far enough away that they are no different than a single point mass, I don't see why the orbit would be unstable. If the planet were at 1 or 2 AU's, that would be different.
Maybe these type of planets arent THAT rare...