Astronomers claim to have seen the merger of two black holes within a third
It is not about three black holes colliding at the same moment, but about a three massive black holes in three colliding galaxies that one day, in the far future, probably will collide and about a possible effect that two black holes at a close distance might come together sooner in the presence of a third black hole in the vicinity. So, not even discussing the possibility that they may merge at the same moment.