This looks like the underground dwelling of the Tuatha de Danaan, or "Children of Danu", the mythological mother goddess of the Irish Celts. Creatures such as leprechauns and other Fairy Folk were known to inhabit these places, particularly under the burial mounds, which were also known as the fairy mounds for that reason.
This definition of 3 is found in many religions 3x3 and physics. Physics professor Brian Greene of Columbia, explains it so beautifully for everyday people "The Elegant Universe" The # is 9. In quantum mechanics a particle holds 3 to balance itself, but itself needs balancing w/2 other 3s. Neutron, Positron & Electron, 1 unit. Their mass equals them out in power. Need 2 more to keep the balance to work, how Universe runs on quantum rules. Feng Shui 9 is the real kicker.