A transiting hot Jupiter with two outer siblings orbiting an intermediate-mass post-main-sequence star

Exoplanetary systems with multiple giant planets present an opportunity to understand planet formation, migration processes, and longterm system-wide dynamical interactions. In particular, they provide constraints to distinguish between smooth disk-driven migration or more dynamically excited system evolution pathways. We report the discovery and characterization of a unique multi-planet system hosting three gas giant planets orbiting the post-main-sequence star TOI-375. The innermost planet, TOI-375 b, was initially detected by the TESS mission, and then confirmed with photometric follow-up observations conducted using MEarth and LCOGT, and radial velocity measurements obtained with FEROS and CHIRON. The radial velocity data revealed the presence of two additional planetary candidates, TOI-375 c and TOI-375 d. We find that TOI-375 b is a hot Jupiter with an orbital period of 9.45469 ± 0.00002 days, a mass of 0.745 ± 0.053 MJ, a radius of 0.961 ± 0.043 RJ, and an eccentricity of0.087 ± 0.042. The outer two planets, TOI-375 c and TOI-375 d, are warm-cold and cold Jupiters with orbital periods of 115.5−1.6+2.0 days and 297.9−18.6+28.9 days, and minimum masses of 2.11 ± 0.22 MJ and 1.40 ± 0.28 MJ, respectively. In terms of formation and overall system architecture, the physical properties of TOI-375 b are consistent with the core accretion scenario, while the current configuration of the system could be explained by both disk-driven and high-eccentricity migration scenarios. The discovery of TOI-375 as the first known system hosting three or more fully evolved gas giants, with at least one transiting planet, makes it an excellent candidate for testing formation and migration theories.