Import keyed on multipl linked fields

There is a Table 1 “Responses” that refers to Table 2 “Clients” and Table 3 “items” and has a text field “Response”. I want to import data in to the table “Responses” that has

Client_Name | Item Name | Response

The records already exist in the table and i want to match based on “Client” AND “Item”. However, if i set both to key, each record is identified as new. How can i import in this way?