Why is the three feathers Wales emblem?

Answer 1

The Prince of Wales's feathers is the heraldic badge of the Prince of Wales. It consists of three white feathers emerging from a gold coronet. A ribbon below the coronet bears the motto Ich dien (German for "I serve", a contraction of ich diene). As well as being used in royal heraldry, the badge is sometimes used to symbolise Wales, particularly Welsh Rugby union and Welsh rugby league; however, its use as such is controversial and rejected by many Welsh people as a symbol of the British monarchy rather than Wales.

The badge has no connection with the native Princes of Wales. It is thought to have originated with Edward, the Black Prince, the eldest son of Edward III of England. According to legend, the Black Prince obtained the arms from the blind John I of Bohemia, against whom he fought in the Battle of Crécy in 1346. After the battle, the prince went to the body of the dead king (whom he admired for his bravery) and took his helmet, lined with ostrich feathers. The feathers and the dead king's motto made up the prince's new badge and came to be used by subsequent Princes of Wales. A key factor in the English army's defeat of the French was the use of Welsh archers, it may have been Edward's pride in the men of Wales which led him to use a symbol of their victory as his emblem. The motto in German is "Ich Dien" which is a homophone for the Welsh language phrase "Eich Dyn" meaning "Your Man", which would have helped enamour the young Black Prince to the Welsh soldiers in particular.

This excerpt is taken from the wiki page in the link below.

Answer 2

It derives from the Prince of Wales heraldic badge. Legend has it that at the battle of Crecy Edward the Black Prince took the ostrich lined helmet off the dead King of Bohemia whom he admired and adopted his motto. Welsh archers had contributed greatly to the victory.