You guys need to give yourselves time, you're not going to lose a ton of weight in 3 weeks. The key is consistency and sustainability... going nuts for a week just messes up your body chemistry. Get a healthy calorie deficit every day and watch what happens in 2-3 months.
1. Adjust your diet. Eat lots of fruits and veggies and drink lots of water, try to go for lots of bulky foods with high fiber content if you're used to eating a lot of crap, you need to fill your belly to stay satisfied (apples, oranges, spinach, kale, etc). Eat 6 small meals throughout the day to keep something in your stomach so you dont reach for junk food. If you have a sweet tooth, only allow yourself one treat per day and try to make it something as good as you can (a small handful of honey granola with some m&m's). Try using programs like Livestrong, Fitday, etc to keep track of how many calories you actually eat each day, you might be surprised where you are picking up a lot of excessive calories and can easily cut/replace things.
2. Get in at least 30 minutes of cardio each day, I recommend running at a fast jog pace, as the best return on your time (use a heart rate monitor on various cardio activities and you'll see what I mean).
Add in 1-2 Tabata sessions for intensity. A Tabata is an interval set where you go as hard as you can for 20 seconds (no really, I mean hard, you should be wasted after 20 seconds), take a 10 second rest, then repeat 8 times total (4 minutes). You can download (or make) MP3 files with a fight bell ringing every 20 and 10 second interval, overlaid over some upbeat music... works great. Try to pick 3-4 exercises to cycle through and try to balance pushing and pulling movements (push-ups, pull-ups, situps, burpees, etc). If you can get in two sessions per day, all the better, but keep it sustainable.
Try to target around a 1,000 calorie deficit per day (again track it in a calorie application for a couple weeks and you'll see what changes you need to make to your diet). This is a healthy level that will burn exactly 2 pounds of body fat per week. I've seen this work time and time again. Do this for two months and you will have lost 16 pounds of body fat.