from Wikipedia ...
"The fleur-de-lis is a stylised design of an iris flower which is used both decoratively and symbolically ... As a religious symbol it may represent the Holy Trinity, or be an iconographic attribute of the archangel Gabriel, notably in representations of the Annunciation. It is also associated with the Virgin Mary ...
In the Middle Ages the symbols of lily and fleur-de-lis overlapped considerably in religious art. Michel Pastoureau, the historian, says that until about 1300 they were found in depictions of Jesus, but gradually they took on Marian symbolism and were associated with the Song of Solomon's "lily among thorns" (lilium inter spinas), understood as a reference to Mary.
Other scripture and religious literature in which the lily symbolizes purity and chastity also helped establish the flower as an iconographic attribute of the Virgin.
In medieval England, from the mid-12th century, a noblewoman's seal often showed the lady with a fleur-de-lis, drawing on the Marian connotations of "female virtue and spirituality". Images of Mary holding the flower first appeared in the 11th century on coins issued by cathedrals dedicated to her, and next on the seals of cathedral chapters, starting with Notre Dame de Paris in 1146.
The flowers may be "simple fleurons, sometimes garden lilies, sometimes genuine heraldic fleurs-de-lis". As attributes of the Madonna, they are often seen in pictures of the Annunciation, famously in those of Botticelli and Filippo Lippi. Lippi also uses both flowers in other related contexts: for instance, in his Madonna in the Forest.
The three petals of the heraldic design reflect a widespread association with the Holy Trinity, a tradition going back to 14th century France, added onto the earlier belief that they also represented faith, wisdom and chivalry."