Final answer:
Sir Bedivere throws Excalibur into the lake on his third attempt, complying with King Arthur's command after initially hesitating due to the sword's beauty and value.
Step-by-step explanation:
In the narrative of Morte d'Arthur, it is indeed Sir Bedivere who, on the third attempt, fulfills King Arthur's dying command to throw the sword Excalibur back into the lake.
The first two times Sir Bedivere goes to the lake, he deceives Arthur, hiding the sword instead of tossing it into the water because of its beauty and worth.
It is only after King Arthur questions him and emphasizes the importance of the task that Sir Bedivere truly throws the sword into the lake, where a hand emerges from the water to catch it and pull it under.