>This would work but I am wondering if there could be something simpler:

Yes, very simple...

SELECT Client.Numero
 FROM Client
 LEFT JOIN Invoice ON Invoice.Numero=Client.NoInvoice AND Invoice.SecondCondition=True
 LEFT JOIN Inventory ON Inventory.Numero=Client.NoInventory AND Inventory.SecondCondition=True
WHERE Invoice.Numero IS NOT NULL OR Inventory.Numero IS NOT NULL