SELECT t1.col1, t2.col5
  FROM t1
  JOIN t2 ON t2.tid = t1.tid
  WHERE t2.col3 = 99 AND ...
UNION
SELECT t1.col1, t3.col5
  FROM t1
  JOIN t3 ON t3.tid = t1.tid
  WHERE t3.col4 = 42 AND ...