When specifying the relationship role condition for the list that you provide the user id to, you can expand the top most role (in this case B to C when you select C as the list entity). From there you can 'chain' the relation with A to B, so that the top most entity is A and the lowest entity is C. So in the end the relationship role condition on your list would be AToB.BToC.
You probably want to tick the 'Distinct' checkbox for the list or else you might have duplicates in it.