VB Script for checking division by 3 in Excell

The math rule is that if a sum of the digits of a number can be divided by 3 without remainder, than the number is self can be divided by 3 without remainder. I don't know if this has been proven, but I know for sure that it works. So if you calculate the sum of the digits of a number and it's 3, 6 or 9, you can be sure it can be divided by 3 without remainder. The function I wrote below just sums the digits recursively until there is a single digit. You can do the final if check on your own :D

Function DigitSum(txt As String) As String

    Dim rez As Long

    Dim srez As String

    rez = 0

    'tt = ""

    For i = 1 To Len(txt)

        k = Mid(txt, i, 1)

        'tt = tt & k

        j = CInt(k)

        rez = rez + j

    Next i

    srez = CStr(rez)

    While (Len(srez) > 1)

      srez = DigitSum(srez)

    Wend

    DigitSum = srez

End Function