The third portal in Blox Fruits is the Tiki Outpost portal in the Third Sea, which becomes accessible after defeating the Tyrant of the Skies. The other Third Sea portals are the Mansion and Hydra Island portals, and the Castle on the Sea has three portals total.

Where it leads

  • Castle on the Sea : three portals.
  • Mansion : one portal.
  • Tiki Outpost : one portal.
  • Hydra Island : one portal.

Unlock requirement

To use the Mansion and Hydra Island portals, players must defeat rip_indra True Form first, while the Tiki Outpost portal requires defeating the Tyrant of the Skies.

TL;DR: the “third portal” usually refers to the Tiki Outpost portal in Third Sea, but if you meant the third portal at Castle on the Sea, that area has three portals total.