Mounting (Symetrical) BFI, M3x16's?

[Steiner-SE]

I'm slightly confused installing my symetrical BFIs as the BOM states 2x M3x16 NHSC, my hardware kit comes with 3, but it seems 4 are needed? You place M5 and M3 t-nuts top and bottom, and there are holes for them in the housing, for two sides, that's 4 M5x16 and 4x M3x16. What holes should be empty, should both M3 t-nuts not be placed on the bottom?