3 off meaning three of them are required.
I had always assumed this use, which is not mentioned in my dictionary, was local to the electronics industry.
How about a mistake that perpetuated itself and sloppy editing and -- my favorite hobby horse -- the lack of understandable-by-laymen writing in the technical industries's handbooks and directions.
Harumph!
wow
P.S. "Let us not persevere in the wrong for the sake of consistency"
a quote from one of the lesser known US Presidents whose name escapes me.