Excel cell change event listener

Worksheet_Change.vba

' 1) Open the developer mode
' 2) On the left bar, under `Microsoft Excel Objects` tab open your sheet you want to observe changes 
' 3) Paste the code you find below

Private Sub Worksheet_Change(ByVal Target As Range)
   Dim KeyCells As Range

' The variable KeyCells contains the cells that will
    ' run you code when they are changed.
    Set KeyCells = Range("A1:A3")

    If Not Application.Intersect(KeyCells, Range(Target.Address)) _
           Is Nothing Then

' Runs whenever the KeyCells changed.
        ' Place your code here.

    End If
End Sub