VBA Check of string exists in an Array

Dear all,
I am trying to develop the macro to get Output as mentioned in a attachment
I am done upto code as below but i want only unique values(first 3 letters of each cell) to be added in "sloop" variable.
can anyone please help me to correct the below code..?

The problem is in line code : ElseIf InStr(1, sloop, xSub) = 1

Just in order to get best possible solution, this querry is cross posted @
http://www.ozgrid.com/forum/showthread.php?t=200350

Sub test()
Dim xSub As String
    Dim sloop As Variant
     
     
    For Each cell In ActiveSheet.Range("A2", ActiveSheet.Cells(Rows.Count, "A").End(xlUp))
        If cell.Value <> "" Then
            xSub = Left(cell.Value, 3)
            If sloop = Empty Then
                sloop = xSub
                 'ElseIf InStr(aa, dd) Then
            ElseIf InStr(1, sloop, xSub) = 1 Then
GoTo iteration:
            Else: sloop = sloop & "," & xSub
 
            End If
        End If

iteration:
    Next cell
End Sub