A search for the production of three massive vector bosons in proton–proton collisions is per-formed using data at√s=13 TeV recorded with the ATLAS detector at the Large Hadron Col-lider in the years 2015–2017, corresponding to an integrated luminosity of 79.8 fb−1. Events withtwo same-sign leptons`(electrons or muons) and at least two reconstructed jets are selected tosearch forW±W±W∓→`ν`νqq. Events with three leptons without any same-flavour opposite-sign lepton pairs are used to search forW±W±W∓→`ν`ν`ν, while events with three leptonsand at least one same-flavour opposite-sign lepton pair and one or more reconstructed jets areused to search forW±W∓Z→`νqq``. Finally, events with four leptons are analysed to searchforW±W∓Z→`ν`ν``andW±ZZ→qq````. Evidence for the joint production of three mas-sive vector bosons is observed with a significance of 4.0 standard deviations, where the StandardModel expectation is 3.1 standard deviations.
