I took the Biology, Math 2, and World History subject tests all in one sitting, but I think I did terribly in Biology. Can I cancel the Biology score but keep the other two test scores?
No, you can't cancel an individual subject test score if you took multiple tests on the same date. If you choose to cancel your Biology score, you will have to cancel your scores for Math 2 and World History as well. The only (very rare) exception to this policy would be if your calculator or listening device malfunctioned while taking a Math or Language with Listening subject test.
My advice is not to cancel your scores. You spent a fair amount of money taking those three tests, and I assume you also put a lot of effort into preparing. There's no reason to throw away your presumably good Math 2 and World History scores. Remember, you can use Score Choice to choose which subject test scores you actually send to colleges. Assuming you're applying to schools that recommend or require only two subject tests, you could just send your Math 2 and World History scores and leave out Biology. I'd only advise you to retake Biology if you're applying to Georgetown, which requires three subject tests, or if you plan on becoming a Biology major and want to demonstrate a strong interest in the subject.