A preprint version of the article is available at arXiv ePrint: 2201.05578 (https://arxiv.org/abs/2201.05578). Copyright © CERN, for the benefit of the CMS Collaboration. A search for heavy neutral leptons (HNLs), the right-handed Dirac or Majorana neutrinos, is performed in final states with three charged leptons (electrons or muons) using proton-proton collision data collected by the CMS experiment at √s = 13 TeV at the CERN LHC. The data correspond to an integrated luminosity of 138 fb−1. The HNLs could be produced through mixing with standard model neutrinos ν. For small values of the HNL mass (