  We report the Transiting Exoplanet Survey Satellite discovery of three small
planets transiting one of the nearest and brightest M dwarf hosts to date,
TOI-270 (TIC 259377017; K-mag 8.3; 22.5 parsec). The M3V-type star is transited
by the super-Earth-sized TOI-270 b (1.247+0.089-0.083 R_earth) and the
sub-Neptune-sized exoplanets TOI-270 c (2.42+-0.13 R_earth) and TOI-270 d
(2.13+-0.12 R_earth). The planets orbit close to a mean-motion resonant chain,
with periods (3.36, 5.66, and 11.38 days) near ratios of small integers (5:3
and 2:1). TOI-270 is a prime target for future studies since: 1) its
near-resonance allows detecting transit timing variations for precise mass
measurements and dynamical studies; 2) its brightness enables independent
radial velocity mass measurements; 3) the outer planets are ideal for
atmospheric characterisation via transmission spectroscopy; and 4) the quiet
star enables future searches for habitable zone planets. Altogether, very few
systems with small, temperate exoplanets are as suitable for such complementary
and detailed characterisation as TOI-270.
