These three galaxies are expected to merge over the next billion years. The ensemble is known as SDSS J084905.51+111447.2. Credit SDSS & NASA/STScI
This gravitationally bound triplet of active galaxies is at redshift of z = 0.077, corresponding to a distance of around 1 billion light-years.
They have been studies in the near-infrared, optical, and X-ray portions of the spectrum, and each galaxy shows evidence of a supermassive black hole at its respective center. The largest of these central black holes looks to be a billion times the mass of the Sun.
Thee projected separation on the sky is only 30,000 light-years, so they are well along in the process of merging due to tidal interactions. The merger of two, and then one more, supermassive black holes will lead to some amazing fireworks.
Here’s the kicker: we are seeing this triplet as it was a billion years ago, due to the light travel time. The dynamical time for the merger is estimated at a billion years.
So in fact the final merger could be happening right now, we’ll just have to wait a billion years for the movie of how it all unfolded.