The threefoil sign is an ancient symbol for holiness and divinity. See [image of a blob with three nodes] in Group 26. The threefoil cross [image of a cross trefly, i.e. whose ends terminate in blobs of three nodes] is also known as the cross of Lazarus[.] It was adopted in the Christian symbolism and given a Christian meaning. For the history of the concept of the Holy Trinity, look up [image of Borromean rings] in Group 25.
Source: wiktionary