How to write a custom Excel 2016 function that gets first two characters of cell if numbers?

Function CombineText(rg As Range) As String
    Dim col As Integer
    If rg.Columns.Count = 3 And rg.Rows.Count = 1 Then
        For col = 1 To 3
            If rg.Cells(1, col) = "" Then
                CombineText = CombineText & "00"
            Else
                CombineText = CombineText & Left(rg.Cells(1, 1), 2)
            End If
            If col < 3 Then
                CombineText = CombineText & "-"
            End If
        Next col
    End If
End Function

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