Private Sub Worksheet_Change(ByVal Target As Range)
Dim rng As Range, cel As Range
With ActiveSheet
    Set rng = Intersect(.Columns(3), .UsedRange)
    If Not rng Is Nothing Then
        Application.EnableEvents = False
        For Each cel In rng
            cel.Value = LCase(cel.Value)
        Next cel
        Application.EnableEvents = True
    End If
End With
End Sub