Sub ColonnesDiff()
Dim Rng As Range, C As Integer
Set Rng = ActiveSheet.[A2].CurrentRegion
C = 5
Do
If Rng(1, C).Value <> "Diff" & (C - 5) \ 3 + 1 Then
Rng.Columns(C).Insert
Rng(1, C).Value = "Diff" & (C - 5) \ 3 + 1
Rng(2, C).Resize(Rng.Rows.Count - 1).FormulaR1C1 = "=IF(RC[-2]=RC[-1],""OK"",""NOK"")"
End If
C = C + 3: Loop Until C > Rng.Columns.Count + 1
End Sub