Using a number 3 pencil probably won't cause any problems for you. At PrepScholar last year (June 2015), we had two of our writers try taking the SAT/ACT with a non-wooden-number-2 pencil, and neither of them had scoring issues.
Obviously, this doesn't mean that students should PLAN on taking the ACT with a mechanical or non-number 2 pencil (since the guidelines specifically state you need to use a number 2 pencil); it does, however, mean that if you accidentally used a non-approved pencil, you're probably okay. As a recent article on our blog states, the original reasoning behind requiring number 2 pencils was that the scoring machines couldn't read lighter pencil leads (3 and up), and that darker pencil leads (1) tended to smear more easily. Newer machines, however, have much less difficulty in reading answer sheets - in fact, some machines are so sophisticated that they'll be able to determine which answer bubble is most darkly filled in and use that one. If you have a double-sided answer sheet, then your sheet will be scored using a newer machine (older machines would be confused by the marks on the other side of the page), and so using a non-number 2 pencil (assuming you mark your answer bubbles darkly and thoroughly) is fine.
If you accidentally use a non-number 2 pencil and do have issues getting your score, or you get your score back and it is lower than you expected, you do always have the option for hand-scoring. Before you shell out the $45 for multiple choice handscoring (+ another $35 to get your essay hand scored), though, you might want to do Test Information Release (available for April, June, and December SAT test dates) to make sure your lower score is not just due to your missing a bunch of difficult questions.