Add it as another JOIN. Say Table3 is the name of the 3rd table. For instance :
SELECT T1.src_prod_ofr, T1. source_code, T2.cit_code
FROM Table1 T1
LEFT JOIN Table2 T2
ON T1.src_prod_ofr = T2.src_prod_ofr
JOIN Table3 T3
ON T2.cit_code = T3.cit_code
This would only return rows where Table3's cit_code is the same as Table2's cit_code. To return all records, do another LEFT JOIN instead of the normal JOIN.