Sub TestSQL_Thomas()
    Dim cnn As New ADODB.Connection
    Dim rst As New ADODB.Recordset
    Dim strSQL As String
    Set cnn = CurrentProject.Connection
    strSQL = "SELECT DISTINCT  [Grafik_A] AS [Grafik] FROM [Tabelle_A] " & _
             "UNION SELECT DISTINCT [Grafik_B] AS [Grafik] FROM [Tabelle_B] " & _
             "UNION SELECT DISTINCT [Grafik_C] AS [Grafik] FROM [Tabelle_C] " & _
             "ORDER BY [Grafik];"
    rst.Open strSQL, cnn, adOpenKeyset, adLockOptimistic
    Do While Not rst.EOF
        Combo1.AddItem rst![Grafik]
        rst.MoveNext
    Loop
    rst.Close
    cnn.Close
    Set rst = Nothing
    Set cnn = Nothing
End Sub