Thanks for your replies! I know the goal I've set for myself is rather high, and I'm not planning on starving myself or exercising myself to death. I'm truly not expecting miracles. I've set this goal mostly as a sort of kick-start. I need something to push me out of this cycle of terrible eating and sloth that I've been in. A serious goal requires serious work and a lifestyle change, which takes more than three weeks. To tell you the truth, I'd probably be very happy to lose 12 pounds in three weeks...but I can always push for just a little bit more...
Thank you both for the warm welcome! I'll let you know how it goes.