Things that are not written anywhere, but that judges have to know (and even to master) anyway. These include judging techniques, policies, etc. Learning these is usually difficult, and my experience is that there are only two ways of learning these:
a) if you know that there is something you have to learn, you are in the right way: ask other judges about it, either in a tournament, if you have the opportunity, or using the judges' email list. Something that surprises me is the huge amount of rules questions asked there, compared to the number of penalties/policy/organization questions, particularly if we take into account that there is a document, the Comprehensive Rules, that contains everything about game rules. Well, if you have any kind of question, don't hesitate to ask there: after all, almost all the judges (that includes L3+ judges) may read your question and give you their opinion. b) there are things that you didn't even know that existed. There seems to be only one way of learning those: go to a tournament, and if you make a mistake in front of an experienced judge, he will tell you and help you to correct it. Yes, it is hard, but don't be afraid of making mistakes: after all, you are learning, and you'd rather find out before testing, wouldn't you?
Being the Head Judge of medium tournaments (50-100 players) is a very good way of learning tournament organization; try to head-judge PTQs regularly if possible. Also, you will have to behave professionally, since you will be the maximum representative of the DCI. If you have other judges in your tournament, keep an eye on them, so you can give them some feedback about their performance Teaching gives you a big opportunity to practice communication and diplomacy, two skills that are very important for mentoring and certification, one of the main tasks of a L3 judge.
If you have the chance to attend to a Grand Prix, don't miss it. Working at those events is an invaluable experience towards L3, particularly if you are a Team Leader. Usually, the huge number of players makes the tournament understaffed, which means not only a lot of work, but also that some of the judges there will have next to no experience. It may happen that some of them are not motivated at all, make some mistakes, don't behave in a professional way... and team leaders have to take care of them. In short: you will work under pressure. Remember, your tournaments won't be like this, but you if you work at several Grand Prix you will be prepared for any catastrophe. Also, don't forget that the Head Judge will be an experienced judge; tell him that you are preparing for L3 and he may give you some feedback later.
If you are lucky enough as to have a Pro Tour tournament or similar close to you, or to receive sponsorship to attend, don't hesitate to go. Just being there, watching how the best judges in the world act, is great. I don't have to say more. I have attended to three of these events in a few months (PT Barcelona, European and Worlds) and I definitely recommend to work at one of those tournaments at least once. You will learn there how to behave in a really professional way towards the players, how to handle the (few) situations that happen there... the high profile of players and judges ensures a smooth tournament, so you will be able to talk to many of those judges, learn their points of view, and so on, without the pressure of high responsibility.
In general, working at as many tournaments as you can (the bigger the better) and talking to other judges is the way to learn all those things you need.
Am I ready to take the L3 test?
If you think you are not very good at any of the things I mentioned before, you should try to improve by gaining experience. It is better if you ask your local L3 judge, or the Head Judge of the tournament, to give you some feedback about how you do at tournaments.
If you think you are ready, then there is still one more thing you have to do: you need a recommendation by a L3 judge, so you should contact the closest one to you and tell him. He will probably ask you to judge one or more tournaments with him to see how well you do. This is an important step, since we may point out some weaknesses you didn't know you had. If this is the case, some more practice should give you what you need.
To summarize, try to be as sure as possible of being ready for the test. Remember, there is no point in trying if you don't think you will succeed. Don't be afraid of asking, don't be afraid of making mistakes. You will learn not only from reading, but also (and mainly) from experience. Bigger tournaments are a must, not only because of the experience but also because of the help and feedback other judges can give you. If possible, try to have a L3 judge mentoring you; he will see your strengths and weaknesses better than yourself.