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Private Sub Worksheet_Change(ByVal Target As Range)
With Target
     If .Count > 1 Or .Item(1) = "" Then Exit Sub
     If Not Intersect(Range("a3:c3"), .Cells) Is Nothing Then
        .Cells(-1, 1) = Val(.Cells(-1, 1)) + Val(.Value)
        .ClearContents
     End If
End With
End Sub

A3:C3 >> A1:C1