<p>If I go ahead and sign up for three subject tests, can I change my mind which ones I want to take on test day? Can I just fill in the box next to a different test? Can I just not take one of the tests?</p>
<p>You can change at will, provided you're not changing to or from a language with listening test. You can take one, two or three tests, but you won't get your money back for the tests you don't take. If you had just signed up for 1 test, you could still take 3, but the CB would bill you for the extra 2 tests (I think that's how they handle the extra $. Can anyone else confirm that?)</p>
<p>Yep, they bill you $8 for extra tests that you didn't sign up for. Changing the subject doesn't matter like PeteSAT said.</p>