Jane,

First lose the aliases from the fields in teh index keys, they will give you trouble later on.

Second; the primary key for table 1 is Id and table 2 is related to it. Thus you want the PK in table 1 to be also in table 2 as a Foreign key. Table 3 is related to table 2 and should contain the PK from table1 and the PK from table 2. Therefore Table 3 should have the PK from table 1 as a field in it and you should use that as the field in Table 3's index key expression.

You don't want an index on one table that refers to a field in another table.