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We report the discovery by the HATSouth exoplanet survey of three hot-Saturntransiting exoplanets: HATS-19b, HATS-20b, and HATS-21b. The planet hostHATS-19 is a slightly evolved V = 13.0 G0 star with [Fe/H] = 0.240, a mass of1.303 Msun, and a radius of 1.75 Rsun. HATS-19b is in an eccentric orbit (e =0.30) around this star with an orbital period of 4.5697 days and has a mass of0.427 Mjup and a highly inflated radius of 1.66 Rjup. The planet HATS-20b has aSaturn-like mass and radius of 0.273 Mjup and 0.776 Rjup respectively. Itorbits the V = 13.8 G9V star HATS-20 (Ms = 0.910 Msun; Rs = 0.892 Rsun) with aperiod of 3.7993 days. Finally, HATS-21 is a V = 12.2 G4V star with [Fe/H] =0.300, a mass of 1.080 Msun, and a radius of 1.021 Rsun. Its accompanyingplanet HATS-21b has a 3.5544-day orbital period, a mass of 0.332 Mjup, and amoderately inflated radius of 1.123 Rjup. With the addition of these three verydifferent planets to the growing sample of hot-Saturns, we re-examine therelations between the observed giant planet radii, stellar irradiation, andhost metallicity. We find a significant positive correlation between planetequilibrium temperature and radius, and a weak negative correlation betweenhost metallicity and radius. To assess the relative influence of variousphysical parameters on observed planet radii, we train and fit models usingRandom Forest regression. We find that for hot-Saturns (0.1 < Mp < 0.5 Mjup),the planetary mass and equilibrium temperature play dominant roles indetermining radii. For hot-Jupiters (0.5 < Mp < 2.0 Mjup), the most importantparameter is equilibrium temperature alone. Finally, for irradiated higher-massplanets (Mp > 2.0 Mjup), we find that equilibrium temperature dominates ininfluence, with smaller contributions from planet mass and host metallicity.

HATS-19b, HATS-20b, HATS-21b: Three Transiting Hot-Saturns Discovered by the HATSouth Survey