Yes, Premier, 1K & GS check-in is on Level 3 at EWR Terminal C, and has been for some years (with only a relatively short period where they moved it to Level 2 during 2020 for 'Covid-19' reasons, and once for construction, IIRC.)


Keep in mind, also, that according to UNITED: "As of May 2, you can only access and exit Newark’s Terminal C from Door 1 on the middle and lower levels (Departures and Arrivals). The other doors on those levels, as well as the doors on the upper level (Level 3) of Terminal C, will be closed until further notice."
...from: https://www.united.com/ual/en/us/fly.../maps/ewr.html