I competed in boxing, wrestling, jiu-jitsu and the like. I've cut weight many times . . . everything from just not eating the morning of the competition, taking laxatives (don't do this), running laps and skipping in a plastic garbage bag an hour before weigh in, to eating very clean for three or four weeks beforehand. Three pounds is not a hell of a lot, so don't be too worried. I'm assuming you have a very accurate scale that you're using? (You don't want to go through all the effort of cutting weight to weigh in eight lbs under on competition day because your scale is heavy - been there, done that).
If the weigh-ins are very close to competition, you will end up hurting your performance more than helping it if you cut much weight via dehydration (For myself, any lower than 1% of normal bodyweight via sweating it out). Your body can only absorb so much liquid each hour . . . and competing dehydrated does affect your strength. The most that I would do in your situation is nearly completely cut salt out of my diet the day before . . . your body will retain less water overnight and into the next day by doing this, which can knock a few lbs off. Your body is always lighter in the morning (when I was competing at 195, I'd be 195 in the morning and 200 in the evening before bed). Try to get into a habit of taking a crap first thing in the morning to shave off a few more ounces. Weigh yourself every time you eat something and drink something on the morning of the competition to ensure you don't go over the limit.
Remember to try all this stuff a month or two before the tournament so that you know exactly how your body will react.
If you had more weight to drop, it's just a matter of getting into shape. Start eating very clean. Reduce your bad carb intake by a lot and increase your protein consumption to reduce muscle loss (you'll lose some no matter what you do, but this limits it). It's important to keep weighing yourself a couple times each day when doing this . . . because it's easy to lose too much weight which will impact your strength. If you're dropping too far, eat more. Keep your work load normal to maintain your muscle mass right up until the last week or so where you start tapering so that you go into the tournament fresh.
PS - It's a whole different scenario when they give you half a day or more between weigh in and competition though . . . shit gets pretty miserable. Close weigh-ins are my preference for that reason.
PPS - After a few dozen tournaments where I cut weight heavily, it really stopped being worth it. You can be five or six lbs under the max for your category and still rock the competition . . . and it's a lot less stress this way.