  Nearly one-third of the sources in the $Fermi$-LAT catalogs lack a lower
energy counterpart, hence being referred as unidentified/unassociated gamma-ray
sources (UGSs). In order to firmly classify them, dedicated multifrequency
follow-up campaigns are necessary. These will permit to unveil their nature and
identify the fraction that could belong to the class of active galaxies known
as blazars that is the largest population of extragalactic $\gamma$-ray
sources. In $Fermi$-LAT catalogs there are also gamma-ray sources associated
with multifrequency blazar-like objects known as Blazars Candidates of
Uncertain type (i.e., BCUs) for which follow up spectroscopic campaigns are
mandatory to confirm their blazar nature. Thus, in 2013 we started an optical
spectroscopic campaign to identify blazar-like objects potential counterparts
of UGSs and BCUs. Here we report the spectra of 31 additional targets observed
as part of our follow up campaign. Thirteen of them are BCUs for which we
acquired spectroscopic observations at Observatorio Astrof\'isico Guillermo
Haro (OAGH) and at Southern Astrophysical Research Observatory (SOAR)
telescopes, while the rest has been identified thanks to the archival
observations available from the Sloan Digital Sky Survey (SDSS). We confirm the
blazar nature of all BCUs: three of them are in blazar of quasar type (BZQs)
while the remaining ones can be spectroscopically classified as BL Lac objects
(BZBs). Then we also discovered 18 BL Lac objects lying within the positional
uncertainty regions of UGSs that could be their potential counterparts.
