Camera Ready in 3 Weeks?
I'm preparing for a photo shoot now and I'll share some tips below. But first, know that if your shoot or event is in three weeks, you should be in pretty good shape already. While you can make significant changes in that short period of time, you can't do miracles.
When you have such a short amount of time to get ready, you can't hold anything back. Especially if you aren't in a condition pretty close to what you want to look like. So if you only have a few weeks, there's no way to get in awesome shape using a moderate approach. Especially considering that the third week will be devoted to the peaking process.
Will I Lose Muscle?
A lot of people are afraid of losing muscle by doing too much and eating too little. But there's no way you'll lose muscle if you keep training hard and keep your protein intake high. Not in three weeks.
You may feel flat, and look smaller in your clothes, but this is to be expected. Why? Because you'll lose muscle glycogen, water, and intramuscular triglycerides which will make you look deflated for a large part of the process. Don't panic and screw it up. When you peak (more on that below), your muscles will pop.
Losing muscle is a protective strategy your body uses to allow you to survive. But it's a last resort. The first thing your body will do is make you lazy. You'll subconsciously reduce how much you're moving and how much effort you use in everyday life.
Next, your body will give you cravings and hunger pangs. If that doesn't work and you're able to fight off hunger, the next thing it'll do is decrease the T4 to T3 conversion (T4 is pretty much inactive and T3 is the thyroid hormone that has the highest impact on increasing metabolic rate). When that happens you'll start to feel colder. If that still doesn't work, then you might risk losing muscle.
But this won't happen in two, three, or even eight weeks (if you keep training hard) and it's not likely to happen until you're at a true 8% body fat or lower. A lot of people screw up the process by panicking when they feel flat.
When I have to get in shape quickly, I do two training sessions a day: A whole body workout in the morning, and a hypertrophy workout in the evening.
In the first session, I focus on slightly lower reps (around 6 per set) using mostly compound movements. In the second session, I either train the pulling or pushing muscles. Pulling includes hamstrings; pushing includes quads. These sessions are mostly pump work. Higher volume, lower rest, more isolation, machine and pulley work.
Yes, that's a lot. Yes, every muscle is hit super often. But in the short run it favors muscle retention because you're constantly requiring your muscles to produce force so the body is less likely to dump it. In the long run it's not sustainable because cortisol will become chronically elevated, but for two weeks (remember the third week is the peak) you'll be fine.
Diet-wise you also have to go hard. Heck, Paul Carter, who's monstrous, dieted down with 1,800 calories, sometimes less. Arnold dieted down at around 1,500 calories for his whole prep! Why would you need 2,500, especially for a very brief period? Don't give me the metabolic damage excuse. You will not create any metabolic damage in two to three weeks.
The two approaches you can use are either the Velocity Diet® Plan (which I've used to get my fat loss phase started for 3 weeks), or the good old get-ripped approach from the 60s and 70s: eat only white fish and broccoli, salad, celery and cucumbers 4-5 times a day.
You may feel like crap, but it's only two-three weeks. Extreme results require an extreme approach, especially if you don't have the benefit of using metabolism-increasing drugs like cytomel or clenbuterol.
For the last week, I wouldn't change the training. As for the day prior to the shoot or event, the only different thing I'd do is work the one or two body parts you want to emphasize the most. Two sessions during the day, both to create some local inflammation making those muscles look bigger, but also to increase glycogen replenishment is those muscles.
I'd also use that last week to continue losing fat, so diet-wise don't change anything until two days out.
Two days out, add around 50 grams of carbs four times per day in the form of potatoes, sweet potatoes, rice, and the like. The day prior to the event increase carbs to around 100 grams four times a day. Keep the same sources but also add pineapple.
On that day also take in a tablespoon of glycerol/glycerine with every meal. Take a natural diuretic three times during the day and cut water at around 6 PM (or greatly reduce it).
The morning of the event, use your look to determine how much carbs you need. Have 50 grams if you look good, 100 grams if you're a bit flat, or 150 grams if you're really flat, and none if you're holding some water. Also go with a tablespoon of glycerin.
There's more to it than that, but that should get you in very good shape if you were not too far off to start with.