In my ZC days I basically ate the following:
breakfast (if I ate it at all, I'm skinny as hell so I don't fast very well) Hamburger or steak, unseasoned
dinner: hamburger or steak
I quickly lost any desire for seasonings. I don't know if this is just me or a general thing but when all you're eating is a steak, salt and pepper quickly stop being compelling. Don't know why.
Cook it rare to preserve nutritional value (although that might just be superstition as some of the zeroing in on health folks claim/prove through eating only well done meat). Eat a lot of fat. Oh, LISTEN to your appetite! Don't eat unless you're hungry and when you do eat stop when you feel full.
You probably don't have to worry about iodine for a 3 week trial, or really much of anything given the short timeframes. The only warning I would have is that if you are not digestively used to a high fat diet, or are currently not firmly in ketosis, you might be in for a rough ride. This will pass as your body adjusts, providing you are otherwise normal, and it may persist for the entire three weeks of your experiment. I was ZC/VLC for about three months before my digestion came under control, but I also have Crohn's so I'm probably a bad barometer. Just be aware of the possibility.
Otherwise, have fun. ZC can be a liberating thing if you're the sort who just wants to refuel and move on with whatever else you were doing - you always know what you're going to eat, your body will tell you when to eat, and it's pretty easy (and quick!) to cook a decent rare steak with some practice.
edit: Comment reminded me to note this: http://www.dirtycarnivore.com/Lex.html
Lex Rooker has eaten nothing but raw beef parts for the better part of four years, and he ate cooked meat for a year before that.
The biggest danger you face is not having a health problem in three weeks on this diet. Your biggest danger is drawing incorrect conclusions due to three weeks not being enough time to really judge your response. As I mentioned, it took months for me to adapt, and I'm very glad that I stuck it out - if I had set out to do a three week experiment instead of a one year experiment, I would have failed rather than succeeded.