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The extragalactic $\gamma$-ray sky is dominated by emission from blazars, apeculiar class of active galactic nuclei (AGNs). Many of the $\gamma$-raysources included in Fermi -Large Area Telescope Third Source catalog (3FGL) areclassified as a blazar candidate of uncertain type (BCU) because there is nooptical spectra available in the literature to confirm their nature. In 2013 westarted a spectroscopic campaign to look for the optical counterparts of theBCUs and of the Unidentified $\gamma$-ray Sources. The main goal of ourinvestigation is to confirm the blazar nature of these sources having peculiarproperties as compact radio emission and/or selected on the basis of theirinfrared (IR) colors. Whenever possible we also determine their redshifts. Herewe present the results of the observations carried out in the Northernhemisphere in 2013 and 2014 at Telescopio Nazionale Galilleo (TNG), Kitt PeakNational Observatory (KPNO) and Observatorio Astron\'omico Nacional (OAN) inSan Pedro M\'artir. In this paper we describe the optical spectra of 25sources. We confirmed that all the 15 BCUs observed in our campaign andincluded in our sample are blazars and we estimated the redshift for 3 of them.In addition, we present the spectra for 3 sources classified as BL Lacs in theliterature but with no optical spectra available to date. We found that one ofthem is a quasar (QSO) at a redshift $z$ = 0.208 and the other 2 are BL Lacs.Moreover, we also present 7 new spectra for known blazars listed in theRoma-BZCAT having an uncertain redshift or being classified as BL Laccandidates. We found that one of them, 5BZB J0724+2621 is a `changing look'blazar. According to the spectrum available in the literature it was classifiedas a BL Lac but in our observation we clearly detected a broad emission linethat lead to classify this source as a QSO at $z$=1.17.

the astronomical journal

Optical Spectroscopic Observations of Gamma-ray Blazar Candidates. V.  TNG, KPNO, and OAN Observations of Blazar Candidates of Uncertain Type in the  Northern Hemisphere