Can someone show me how to code the conditional statement shown in the
comments below i.e. how to put the "x" in the cell only if the SHIFT ( or
ctrl or alt)key is selected.

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
RowPos = Target.Row
ColPos = 3
' If Shift Key selected then
' Put an "x" in cell row = RowPos and Column = ColPos
ThisWorkbook.ActiveSheet.Cells(RowPos, ColPos) = "x"
'End If
End Sub