The TRIPLE MERGE JOIN hint is a variant of MERGE JOIN. It has three table sources which are merged on equal basis instead of the two in MERGE JOIN. The TRIPLE MERGE JOIN hint instructs the SQL interpreter to use the triple merge join algorithm whenever possible. The triple merge join algorithm can only be used in situations where in all three table sources there is one single field that should be equal in all the resulting rows after evaluating the WHERE condition.
Example
SELECT--(* vendor(SOLID), product(Engine), option(hint) -- TRIPLE MERGE JOIN *)-- TAB1.I FROM TAB1, TAB2, TAB3 WHERE TAB1.I = TAB2.I AND TAB2.I = TAB3.I;