I am having trouble trying to combine three files to be averaged. I am not so sure how to even start. I have three files

"nday1.06.nc , nday1.07.nc, nday.08.nc"

each with the variables

"filling on), ('SST',  float32 SST(time, nlat, nlon) long_name: Surface Potential Temperature units: degC coordinates: TLONG TLAT time grid_loc: 2110 cell_methods: time: mean time: mean time: mean _FillValue: 9.96921e+36 missing_value: 9.96921e+36 unlimited dimensions: time current shape = (1, 2400, 3600) 

I just need to average the SST variables and then an output file with the averages. I need to do this using linux NCO operators for netCDF4 files.