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We present ground and space-based photometric observations of TOI-270(L231-32), a system of three transiting planets consisting of one super-Earthand two sub-Neptunes discovered by TESS around a bright (K-mag=8.25) M3V dwarf.The planets orbit near low-order mean-motion resonances (5:3 and 2:1), and arethus expected to exhibit large transit timing variations (TTVs). Following anextensive observing campaign using 8 different observatories between 2018 and2020, we now report a clear detection of TTVs for planets c and d, withamplitudes of $\sim$10 minutes and a super-period of $\sim$3 years, as well assignificantly refined estimates of the radii and mean orbital periods of allthree planets.  Dynamical modeling of the TTVs alone puts strong constraints on the massratio of planets c and d and on their eccentricities. When incorporatingrecently published constraints from radial velocity observations, we obtainmasses of $M_{\mathrm{b}}=1.48\pm0.18\,M_\oplus$, $M_{c}=6.20\pm0.31\,M_\oplus$and $M_{\mathrm{d}}=4.20\pm0.16\,M_\oplus$ for planets b, c and d,respectively. We also detect small, but significant eccentricities for allthree planets : $e_\mathrm{b} =0.0167\pm0.0084$, $e_{c} =0.0044\pm0.0006$ and$e_{d} = 0.0066\pm0.0020$. Our findings imply an Earth-like rocky compositionfor the inner planet, and Earth-like cores with an additional He/H$_2$Oatmosphere for the outer two. TOI-270 is now one of the best-constrainedsystems of small transiting planets, and it remains an excellent target foratmospheric characterization.

Transit Timing Variations in the three-planet system: TOI-270