The Key of Three Parts is a very essential item in Ultima IV, where it is needed to unlock the chamber of the Codex deep within the Abyss. The key is split in three parts corresponding to the Principles of Truth, Love and Courage.
The Stranger found the three key parts by venturing into the altar rooms of the Three Principles deep within the dungeons. There, the hero used the Virtue Stones, representing the Eight Virtues, which corresponded with the principle of the altar to get a key part. Having done this three times, the Stranger put together the key, and later used it in the Abyss to gain access to the Codex of Ultimate Wisdom.
Later, in Ultima Underworld I, the key had to be found again, because it was the only way to enter the chamber of virtue which held the Slasher of Veils at the bottom of the Abyss, which is likely where the Codex was originally stored.