Theoretically, CV3 BE is a generic background error statistics file, which can be used for any case. It is quite straightforward to use CV3 in your own case. To use the CV3 BE file in your case, set cv_options=3 in &wrfvar7 in namelist.input in your working directory, and use the be.dat is located in WRFDA/var/run/be.dat.cv3.