Private Sub Worksheet_Change(ByVal Target As Range)
If Not Intersect(Target, Range("a1:a3")) Is Nothing Then
    v = Target.Value
    With Application
        .EnableEvents = False
        .Undo
        Target.Value = Replace(Left(Target.Text, Len(Target.Text) - Len(v)) & v, ",", ".")
        .EnableEvents = True
    End With
End If
End Sub

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
  If Not Intersect(Target, Columns(1)) Is Nothing Then
    Application.EnableEvents = False
Dim iCell As Double
Dim iDrob As Double
    iCell = Target.Value * 100
    iCell = Fix(iCell)
    iDrob = Application.InputBox("Введите три цифры замены", , Type:=1) / 1000
    iCell = (iCell + iDrob) / 100
    Target = iCell
  End If
    Application.EnableEvents = True
End Sub