I am playing Volibear legend. I use here to help to play Towering Combatant while defending. Does this trigger the Volibear legend ability?

No, playing Towering Combatant in this situation will not trigger Volibear’s legend ability.

Here is the breakdown of why:

1. Volibear’s Condition: According to the Volibear legend card, his ability triggers "When you play a [Mighty] unit." A unit is defined as Mighty only if it has 5 or more Might (Rule 708).
2. Towering Combatant's Stats: Towering Combatant has a printed Might of 3. While it has an ability that grants it "+2 Might while I'm a defender," this is a passive ability (effectively an aura-style effect that applies under specific conditions).
3. Timing and Checking: When you play a unit, Volibear checks its Might value at the moment it enters play. Because Towering Combatant enters the battlefield as a 3 Might unit, it does not meet the "Mighty" (5+ Might) condition at the exact moment it is played.
4. Reference: As established in RiftJudge FAQs (e.g., FAQ 6972 and 6020), units that only become Might 5+ after entering play—through passive stat bonuses, combat cleanup, or triggered effects—are not considered "Mighty" at the time they are played, and therefore do not trigger Volibear's legend ability.

Even though you are defending when you play it, and its passive ability will eventually make it Might 5, that increase happens too late for Volibear to register it as a Mighty unit upon being played.