That looks like it might belong in the select statement:
SELECT id, col1, col2, col3, (CASE WHEN table3.col3 IS NULL THEN table2.col3 AS col4 ELSE table3.col3 as col4 END)
FROM table1
LEFT OUTER JOIN table2
ON table1.id = table2.id
LEFT OUTER JOIN table3
ON table1.id = table3.id