简单代码：

Private Sub Worksheet_Change(ByVal Target As Range)
  If Target.Address <> [A1].Address And Target.Address <> [A2].Address Then Exit Sub
  If IsEmpty(Target) Or Not IsNumeric(Target) Then Exit Sub
  [A3] = [A3] + Target.Value
End Sub

这张照片应该会有帮助，我想。

请使用最新代码。
如果你需要对3个单元格求和，e。例如A4 = A1 + A2 + A3：

Private Sub Worksheet_Change(ByVal Target As Range)
  If Target.Address <> [A1].Address And Target.Address <> [A2].Address _ 
    And Target.Address <> [A3].Address Then Exit Sub
  If IsEmpty(Target) Or Not IsNumeric(Target) Then Exit Sub
  [A4] = [A4] + Target.Value
End Sub

如果你需要有两个三胞胎的细胞，e。例如A3 = A1 + A2和B3 = B1 + B2：

Private Sub Worksheet_Change(ByVal Target As Range)
  If Target.Row > 2 Then Exit Sub
  If IsEmpty(Target) Or Not IsNumeric(Target) Then Exit Sub
  If Target.Column = 1 Then
    [A3] = [A3] + Target.Value
  ElseIf Target.Column = 2 Then
    [B3] = [B3] + Target.Value
  End If
End Sub