Explanation:

The most appropriate option to fill in the blank is: 2. foiled

Complete sentence:

Thus, foiled of their purpose, the three knights tried another plan.

Explanation:

"Foiled of their purpose" means their plan or attempt was thwarted or blocked, which fits perfectly with the idea that they had to try another plan.

Involved doesn't fit the context.

Toiled means worked hard, but doesn’t convey the sense of failure.

Failed is similar in meaning, but "foiled" is the more formal and contextually elegant word choice, often used in literary or narrative writing.