  A search for supersymmetry in events with two or three low-momentum leptons
and missing transverse momentum is performed. The search uses proton-proton
collisions at $\sqrt{s} =$ 13 TeV collected in the three-year period 2016-2018
by the CMS experiment at the LHC and corresponding to an integrated luminosity
of up to 137 fb$^{-1}$. The data are found to be in agreement with expectations
from standard model processes. The results are interpreted in terms of
electroweakino and top squark pair production with a small mass difference
between the produced supersymmetric particles and the lightest neutralino. For
the electroweakino interpretation, two simplified models are used, a wino-bino
model and a higgsino model. Exclusion limits at 95% confidence level are set on
$\widetilde{\chi}^0_2 / \widetilde{\chi}^\pm_1$ masses up to 275 GeV for a mass
difference of 10 GeV in the wino-bino case, and up to 205 (150) GeV for a mass
difference of 7.5 (3) GeV in the higgsino case. The results for the higgsino
are further interpreted using a phenomenological minimal supersymmetric
standard model, excluding the higgsino mass parameter $\mu$ up to 180 GeV with
the bino mass parameter $M_1$ at 800 GeV. In the top squark interpretation,
exclusion limits are set at top squark masses up to 540 GeV for four-body top
squark decays and up to 480 GeV for chargino-mediated decays with a mass
difference of 30 GeV.
