I would think if your definite chief aim is achievable in 3 months, it is not big enough. Usually, the definite chief aim should seem unrealistic and unachievable in your current circumstances. You make that your life purpose, meaning you think and breathe it every day making success inevitable. The reason why most people fail to realize their dreams is because they only chase the things they want when they feel like it or when they feel motivated. But the people who succeed are those who think about their dreams every day.
However, this article needs updating. It is very easy for your chief aim to become nothing more than wishful thinking. To avoid this, you should break it down into smaller goals like the 3 months goals you mentioned. Once you do that, you should create habits that will help you make daily progress towards achieving these mini-goals. I recommend you read this – https://ofpad.com/how-to-form-a-habit/ article. If you make pursuing your goals a habit, you end up achieving your definite chief aim in the long run.
For example, if you want to be a best selling author of a book, it is a no brainer that you need to write books. But if you want to be an author today and a celebrity chief tomorrow and a mixed martial arts champion the day after, then you are not going to get anywhere with your book. This is the problem most people face and your definite chief aim is the first step making you focus all your energy and efforts into one thing. Only after that can you set sub-goals. In the best selling author example, your sub goals will be something like create a plot, create character back story and things like that. These are sub-goals that help you achieve your chief aim of becoming a best selling author. Then you create a habit to write 30 minutes every day. With time you will have your book done. Your first book might not take you to the best seller list, but repeat the process enough number of times and you hone your skill and you increase your chances. Irrespective of what the actual end outcome is, you will be much better off than the person who wants many different things but ended up doing nothing.
I will update this article in a few weeks to help increase clarity.
Let me know if this clarifies.