Adding value of two cells to get result in 3rd cell

[DocAElstein]

That macro is OK, if it works for you. It will recalculate everytime you select any cell anywhere in the worksheet. So it is a little inefficient, The macro will be run many times when it is not needed to run.

This might be a bit better,

Private Sub Worksheet_Change(ByVal Target As Range)
 If Target.Address = "$B$3" Or Target.Address = "$E$3" Then
  Let Application.EnableEvents = False
  Let Range("C8").Value = Range("B3").Value + Range("E3").Value
  Let Application.EnableEvents = True
 Else
 
 End If
 
End Sub

But you can use the one you prefer