Based on your inputs your code will be like as mentioned below..
BEGIN
// Add appropriate WHERE Condition
SELECT MAX(id) INTO maxId FROM table2;
// Add appropriate WHERE Condition in SELECT statement mentioned below
FOR i IN (SELECT col1,col2 FROM table1)
LOOP
INSERT INTO table3 VALUES (maxId, i.col1, i.col2);
END LOOP;
END;
Please note , You need to do exception handling separately.