A team of astronomers, led by Graduate Student Ryan Pfeifle of George Mason University (GMU) in Fairfax Virginia, used observations spanning the electro-magnetic spectrum to provide the strongest evidence to-date of the existence of three Active Galactic Nuclei (AGN) residing in a merging galaxy.  
Beginning with a sample of hundreds of likely merging galaxies identified by the Galaxy Zoo Citizen Science Program and using thermal infrared observations from NASA’s Wide-field Infrared Survey Explorer (WISE), Pfeifle and his team identified a few dozen candidate systems which were thought to host two Active Galactic Nuclei (AGN).   AGN are supermassive black holes at least 1 million times more massive than our Sun which are energized by the presence of vast quantities of gas drawn into them by gravitational forces. 
Space-based X-ray observations obtained with the Chandra Observatory Nuclear Spectroscopic Telescope Array (NuStar) found evidence to support the presence of two AGN in most of the merging galaxies.  However, one system appeared to be significantly more unusual than the rest, SDSS J084905.51+111447.2 (shortened to SDSS J0849+1114). Here, the team found X-ray evidence to suggest three AGN resided in this merging galaxy.
In order to provide more definitive support, and rule out competing astrophysical explanations, the team turned to the Large Binocular Telescope Observatory (LBT), currently the world’s largest optical/infrared telescope.  Led by Dr. Barry Rothberg, who is a staff astronomer at LBT and holds a concurrent affiliate faculty position at GMU, optical and infrared spectroscopic observations of SDSS J0849+1114 were obtained using the Multi-Object Double Spectrographs (MODS), and the LBT Utility Camera in the Infrared (LUCIs).
The spectroscopic observations allow optical and near-infrared light to be separated so that atomic and molecular species can be identified.  The MODS and LUCI LBT observations conclusively demonstrated the presence of atomic species which can only exist in conditions found in AGN, as well as the presence of fast flowing gas that could only be powered by these monsters.  Moreover, the spectroscopic data allowed the team to determine the ages of the stars in the host galaxy, further ruling out competing astrophysical explanations for the presence of the three X-ray detections.  
Recent work by the team has expanded their study of this object to radio wavelengths.  “The use of multi-wavelength space- and ground-based observatories shows the synergies needed to discover such unusual, but important astrophysical phenomenon.  There is no way we could have confirmed the existence of a Triple AGN without contributions from all of these observatories,” says Dr Rothberg.
However, the team’s work is far from finished.  Pfiefile notes that the team’s new multi-wavelength technique will allow them to extend their work to finding more triples.  “We already have more candidates that we are actively pursuing. I look forward to LBT once again demonstrating its importance in finding the optical and near-infrared signatures that either confirm or reject the presence of AGN in merging galaxies.”

More details on the findings here with pictures and videos here.