on Trikaya from Wikipedia today:
The Trikaya doctrine (Sanskrit, literally "Three bodies or personalities"; 三身 Chinese: Sānshén, Japanese: sanjin) is an important Buddhist teaching both on the nature of reality, and what a Buddha is. By the 4th century CE the Trikaya Doctrine had assumed the form that we now know. Briefly the doctrine says that a Buddha has three kayas or bodies: the nirmanakaya or created body which manifests in time and space; the sambhogakaya or body of mutual enjoyment which is a body of bliss or clear light manifestation; and the Dharmakaya or Truth body which embodies the very principle of enlightenment and knows no limits or boundaries. In the view of Anuyoga, the 'Mindstream' (Sanksrit: citta santana) is the 'continuity' (Sanskrit: santana; Wylie: rgyud) that links the Trikaya. The Trikaya, as a triune, is symbolised by the Gankyil.
Vajrayana sometimes refers to a fourth body, called the Svabhavikakaya (Wylie: ngo bo nyid kyi sku, THDL: ngo wo nyi kyi ku), meaning essential body. The Svabhavikakaya is simply the unity or non-separateness of the three kayas.
This same symbol----like a 3-headed yin/yang, is seen in the Northwestern corner of France, where the Celts settled. There it symbolizes the three elements: wind, sea, and rock. Fire was weak there.
Last but not least, here's the page of the yoga practice that has attracted my attention thanks to lynnivere: