Table Plant {PID(key), Name}；
Table Provider {VID(key), Company}；
Table Contractor {CID(key), Plant, Provider}；
Table KPI {KID(key), ServiceID, KPI1, KPI2}.

Plant.PID = Contractor.Plant,
Provider.PID = Contractor.Provider,
Contractor.CID= KPI.ServiceID

Private Sub cbxProviderID_AfterUpdate()
ServiceID = DLookup("[CID]", "Contractor", "[Plant]= " & [Forms]![KPI]![cbxPlantID] & " And [Provider] =" & [Forms]![KPI]![cbxProviderID])
End Sub