The Welsh Triads (thing)

• The Three Fair Womb-Burdens of the Island of Britain
• The Three Lovers of the Island of Britain
• The Three Surpassing Bonds of Enduring Love
• The Three Stubborn Men of the Island of Britain
• The Three Peers of Arthur's Court
• The Three Who Could Not Be Expelled from Arthur's Court
• The Three Men of the Island of Britain Who Were Most Courteous to Guests and Strangers
• The Three Fair Maidens of the Island of Britain
• The Three Lively Maidens of the Island of Britain
• The Three Saintly Lineages of the Island of Britain
• The Three Kindreds of Saints of the Island of Britain, By a Welsh Mother
• The Three Blessed Visitors of the Island of Britain
• The Three Bodies which God Created for Teilo
• Arthur's Three Principle Courts
• The Three Knights of Arthur's Court Who Won the Graal, and It Brought Them to Heaven
• The Three Splendid Maidens of Arthur's Court
• The Three Things Which Conquered Lloegr
• The Three Perpetual Harmonies of the Island of Britain
• The Three Fearless Men of the Island of Britain
• The Three Elders of the World
• The Three Men Who Specified Their Sufficiency from Arthur as Their Gifts
• The Three Immense Feasts that Were in the Island of Britain
• The Three People Who Broke Their Hearts from Bewilderment
• The Three Wives Whom Brychan Brycheiniog Had

There are many more, but I'm editing them, sifting through the ones that seem least altered by both Christian and neo-Pagan redaction (i.e., no mentioning of Atlantis).