Final answer:
The three rioters in the Pardoner's Tale do not keep their initial plan or agreement due to their greed.
Step-by-step explanation:
The character flaw that causes the three rioters in the Pardoner's Tale not to keep their initial plan or agreement is their greed. The rioters set out to find and kill Death, but when they stumble upon a hidden treasure, their desire for wealth outweighs their original intention. As a result, they become consumed by their greed and turn against each other, ultimately leading to their downfall.