Hallo
Eine Alternative wäre z.b. auch Select Case
Private Sub worksheet_Change(ByVal Target As Range)
Application.EnableEvents = False
Select Case Not Intersect(Target, Range("A1:C1")) Is Nothing
Case Target.Address = "$A$1"
Union(Cells(1, 2), Cells(1, 3)) = ""
Case Target.Address = "$B$1"
Union(Cells(1, 1), Cells(1, 3)) = ""
Case Target.Address = "$C$1"
Union(Cells(1, 1), Cells(1, 2)) = ""
End Select
Application.EnableEvents = True
End Sub