Today green is the primary color when representing the Irish and St. Patrick's Day. The Harp Harp musical has been played for years in churches and castles in Ireland. The harp appears on the coins and on some flags, as well as the royal coat of arms. The harp is one of the worlds oldest musical instruments and plays a long history in Irish mythology. The number 3 has always been significant throughout history.
Some scholars feel that it goes way back to creation.
The number 3 wasn't unique in St. Patrick's time either. Heavens, seas, and earth and to them the rulers of the world. In Greek mythology there were the 3 Fates and the 3 Muses. The Catholic church makes the sign of the cross as Father, Son and Holy Ghost. And, in many fairy tales the number 3 appears as in 3 bad step sisters in Cinderella and 3 wishes offered by genies.