Final answer:
In 'Morte d'Arthur', Sir Bedivere attempts to throw Excalibur into the lake three times, completing the task on the third attempt, which is an event that occurs thrice.
Step-by-step explanation:
In Morte d'Arthur, an event that occurs three times is the call for assistance from a mystical figure. During King Arthur's final battle, he commands Sir Bedivere to throw his sword Excalibur back into the lake. Bedivere fails to follow through twice, lying to Arthur about completing the task, but on the third attempt, he finally complies.
A hand emerges from the lake to catch Excalibur, suggesting the mystical return of the sword to the Lady of the Lake. This act of throwing the sword happens three times, highlighting themes of loyalty, truth, and the supernatural in the narrative.