The word Trinacria comes from Greek and means “three pointed”, it consists of the word akra (end, point, peak, headland) and the word treis (three), and is the earliest known name of Sicily, due to it’s triangular shape. The ancient symbol of Trinacria depicts the head of the Medusa (a woman’s head with snakes as hair).
The three legs, which are conjoined at the hips and flexed in triangle represent the three capes (the corners of the island when you look at it from bird perspective). Those corners are: Peloro (Punta del Faro, Tip of Faro, Messina: North-East); Passero (Syracuse: South); and Lilibeo (Cape Boeo, Marsala: West) and they form the three points of the triangle. The placement of a woman’s head is further inspired by Native Sicilians, who were left breathless by the beauty of Sicily’s shores and compared them to the beauty of a woman.
But the importance of the triangle within the Sicilian flag is mainly based on the mythological story of Colapesce. You can read the full story HERE.