  A statistical combination of several searches for the electroweak production
of charginos and neutralinos is presented. All searches use proton-proton
collision data at $\sqrt{s}=$ 13 TeV, recorded with the CMS detector at the LHC
in 2016 and corresponding to an integrated luminosity of 35.9 fb$^{-1}$. In
addition to the combination of previous searches, a targeted analysis requiring
three or more charged leptons (electrons or muons) is presented, focusing on
the challenging scenario in which the difference in mass between the two least
massive neutralinos is approximately equal to the mass of the Z boson. The
results are interpreted in simplified models of chargino-neutralino or
neutralino pair production. For chargino-neutralino production, in the case
when the lightest neutralino is massless, the combination yields an observed
(expected) limit at the 95% confidence level on the chargino mass of up to 650
(570) GeV, improving upon the individual analysis limits by up to 40 GeV. If
the mass difference between the two least massive neutralinos is approximately
equal to the mass of the Z boson in the chargino-neutralino model, the targeted
search requiring three or more leptons obtains observed and expected exclusion
limits of around 225 GeV on the second neutralino mass and 125 GeV on the
lightest neutralino mass, improving the observed limit by about 60 GeV in both
masses compared to the previous CMS result. In the neutralino pair production
model, the combined observed (expected) exclusion limit on the neutralino mass
extends up to 650-750 (550-750) GeV, depending on the branching fraction
assumed. This extends the observed exclusion achieved in the individual
analyses by up to 200 GeV. The combined result additionally excludes some
intermediate gaps in the mass coverage of the individual analyses.
