On August 14, 2017 at 10∶30:43 UTC, the Advanced Virgo detector and the two Advanced LIGOdetectors coherently observed a transient gravitational-wave signal produced by the coalescence of twostellar mass black holes, with a false-alarm rate of ≲1 in 27 000 years. The signal was observed with athree-detector network matched-filter signal-to-noise ratio of 18. The inferred masses of the initial blackholes are 30.5þ5.7−3.0M⊙ and 25:3þ2.8−4.2M⊙ (at the 90% credible level). The luminosity distance of the source is540þ130−210 Mpc, corresponding to a redshift of z ¼ 0.11þ0.03−0.04 . A network of three detectors improves the skylocalization of the source, reducing the area of the 90% credible region from 1160 deg2 using only the twoLIGO detectors to 60 deg2 using all three detectors. For the first time, we can test the nature ofgravitational-wave polarizations from the antenna response of the LIGO-Virgo network, thus enabling anew class of phenomenological tests of gravity.