Would like to compile a list of errors that should be avoided during the preparation. May be existing test takers take some time and comment.
I think my biggest mistake (though I did actually pass) was starting the practice exam questions for the AM section too late in my studies. I really bombed the AM section because I hadn’t taken enough time really learning the nuances of how to make an appropriate written response. I think after you get through the individual and institutional portfolio management sections you should start taking a look at some of the past AM exam questions and mock exams or Schweser practice tests. Even if you’re very familiar with the material, it can be difficult to produce a written answer that will satisfy the CFAI graders unless you’ve seen a lot of sample questions and answers.
Fortunately, I had a strong performance in the PM section that got me through, but the AM was miserable for me since I’d really only been drilling those types of questions for a couple of weeks.
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Many comments on L3 preparation will evolve around the fact that the material is not difficult (certainly not as draining as L2 IMO) but that the exam is harder (can’t say I totally agree but I see the point).
This cames back to the fact that the Individual and Institutional IPS are conceptually easy to understand and work through when you see a written answer or blue box, but being able to create this from scratch (as you are asked at least in part to do on pretty much every mock and actual exam) is hard not only because you are starting with a blank piece of paper but also because it is so easy to make simple mistakes.
Totally agree with the idea that you start mocks as early as possible in your preparation but also that you get into the habit of trying to formulate your own answer to examples even when the answer is given below, just so that you get use to the normal calculation errors that seem to occur every time.
Another point I would make is that the old analogy “There is the right answer, the practical answer and the CFA answer” is one that I think L3 candidates should get in their head from the get-go.
I have always been one who likes to study by getting “the big picture” and then knowing that I can probably formulate any answer from this level of knowledge without needing to memorise every formula and list in the CBOK. It has typically worked for me in all study and in particular in clearing L1 and L2 with good results.
In L3 I found that applying your practical, “big picture” application to written answers would normally get you very few marks and with markers simply taking their scoring on the AM from a CFA prepared mark sheet, you want to make sure you know what they want to hear.
I still did not do a lot of memorising for L3 exam (and passed thankfully) but I did have to get my head around what they wanted to hear in a particular type of question. As per the points above, this was only achieved by doing lots and lots (and sometimes multiple times on the same questions/exams) practice questions and mocks.
dont wait until the last minute studying 
For Level III, I did a lot of things right that I learned from II, ie start studying very early on and making sure you know the CFAI books as best as possible particularly every single EOC and Blue Box question. The thing I did not do right is focus too much on the content and not enough on the format of the exam. Luckily I was able to pull through but make sure you give enough focus on mastering the exam format as well as the content.
My advice is to make sure you spend many many hours sitting down in simulated exam conditions and physically writing out practice respones to essay after essay. This mentally trains you such that tackling essay questions is second nature. Otherwise you waste a lot of time during the actual exam trying to figure out how to approach & structure essay responses.
-not using bullet points
-trying to be fancy with writing
-answering more than what’s required (especially with topics you know well)
I totally agree with dave.
Is easier if you use bullet points. Is a short essay so make as simple as possible. You dont have to use sophisticated words or write like a journalist or what not, make it as simple as possible and use the key words.
-thinking level 3 is easier than level 2. Level 2 content was harder, level 3 content is easier but exams is wayyyy harder
-Focus too much on AM. Am is marked by humans and PM is marked by computers. PM has a right answer AM is somewhat subjective. Dont just totally ignore AM but focus on nailing PM more than AM. If there is a main strategy for passing level 3, is for Am minimise your losses or think in a way of getting every single mark that is possible and for PM, aim for perfection.
-Dont aim for perfection especially with individuals IPS. Do the first question as the last question as generally Individuals consumes a lot of time especially since the reward ratio is relatively poor. As you can see from people whom pass and posted their marks, is rare to see them nailing >70 for individuals. Why aim for perfection for something is subjective and risk of not finishing the paper and nailing other straight forward questions like derivatives or fixed income at the end of the paper?
- A cliche but what the heck. Start early writting your AM and time yourself. Have you ever heard people whom pass complaining I started practising too early?
Level 3 is probably the worse result I ever had for the past 3 levels and is the most hours Ive put in.
As I learn more about carrying over study habits to exam days, I wish I would have:
- Brought the same pen to the exam that I used to practice AM answers.
- Drank heavily before the exam like I did when studying every night.
Although I passed, my biggest error was failing to get the timing right in the morning session. Because each subsection has a different number of minutes allocated to it, I found it difficult to get the timing right. In L1/L2, you would know where you were in time by looking at where you were in the number of questions versus the time - in the AM on L3, you can’t do this. Find a method that works for you (someone on AF suggested writing down the time you should finish the question by at the top of the question which I think is eminently sensible).
choking the goose too often