  Young exoplanets can offer insight into the evolution of planetary
atmospheres, compositions, and architectures. We present the discovery of the
young planetary system TOI 451 (TIC 257605131, Gaia DR2 4844691297067063424).
TOI 451 is a member of the 120-Myr-old Pisces--Eridanus stream (Psc--Eri). We
confirm membership in the stream with its kinematics, its lithium abundance,
and the rotation and UV excesses of both TOI 451 and its wide binary companion,
TOI 451 B (itself likely an M dwarf binary). We identified three candidate
planets transiting in the TESS data and followed up the signals with photometry
from Spitzer and ground-based telescopes. The system comprises three validated
planets at periods of 1.9, 9.2 and 16 days, with radii of 1.9, 3.1, and 4.1
Earth radii, respectively. The host star is near-solar mass with V=11.0 and
H=9.3 and displays an infrared excess indicative of a debris disk. The planets
offer excellent prospects for transmission spectroscopy with HST and JWST,
providing the opportunity to study planetary atmospheres that may still be in
the process of evolving.
