If you think the three body problem is a headache, just wait till you try to wrap your math around a collision between four galaxies that has produced three active galactic nuclei. Say hello to Arp–Madore 2339-661. Here’s more from Astronomy Now:

All four galaxies, which are 500 million light years away, are interacting and will eventually merge to form one giant elliptical galaxy. In 2021 a study published in the journal Astronomy and Astrophysics, using data from the Very Large Telescope’s MUSE (Multi Unit Spectroscopic Explorer) instrument and near-infrared observations from the South African Astronomical Observatory, indicated that NGC 7733b contains an active galactic nucleus (AGN) called a Low Ionisation Nuclear Emission-line Region (LINER), while both NGC 7734 and NGC 7733N contain Seyfert AGN. That makes this system, collectively known as Arp–Madore 2339-661, a rare triple AGN. When the galaxies all merge, the supermassive black holes at the heart of these AGN will also merge.

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