I have used your format for Dynamic Combo Boxes with the following substitutions:
SalesRep=Order(OrderCombo)
Customer=Product(ProductCombo)
Order=Operation(OperationCombo)
Successfull:
OrderCombo selects order of choice
ProductCombo diplays only products from that order
BUT.......
OperationCombo fails to limit Operation specific to that product.

Query OrderDrillDownProduct SQL
SELECT OperationT.OperationID, OperationT.OperationAbreviation, OperationT.OperationName, ProductOperationT.ProductID
FROM OperationT INNER JOIN ProductOperationT ON OperationT.OperationID = ProductOperationT.OperationID
WHERE (((ProductOperationT.ProductID)=[Forms]![OrderDrillDownF]![ProductCombo]))
ORDER BY OperationT.OperationAbreviation;

ProdutOperationT does have 2 primary Keys ProductID and OperationID which I used to in another form to limit Operations to a specific Product.

Can you help me get the OperationCombo working?

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Reply from Alex Hedley:

Is [Forms]![OrderDrillDownF]![ProductCombo] acting as a string "[Forms]![OrderDrillDownF]![ProductCombo]" instead of the actual value.
How did you set the Source of the combo, is it to a Table/Query?