“How to” succeed on IB Economics higher level paper 3 | Economics in Plain English
The link above takes you to Jason Welker's website that details a post on exactly what you can expect from a paper 3 exam in May. It's quite long. It's quite numerical. But at this point in time, you should be expecting that.
Paper 3 has the highest average score around the world, but involves calculations from across the entire two year course and all sections of the syllabus. Therefore, it doesn't allow you to focus on just micro or just development.
This might seem a bit overwhelming at first, but you should recognize that many of the formulas are recycled with different names for different situations. For example, line equations are used multiple times and percentage change equations are used multiple times.
The more energy you spend working through the exercises in the book and reviewing the above link now, the easier the exam will be. This is definitely a test where practice and exposure is almost certainly your best bet. Do all the work, ask questions if you get stuck, and you should be as prepared as you possibly can be.
I would also personally take the time to write out all the formulas from across the text so that you can organize them mentally in the way that best works for you. This could be a Google Doc, flashcards, or even taking a list you find from the internet from the many examples that already exist. The point is to keep it handy over the next month, look at it a lot, and do your best to memorize them so you don't have to rely on filling in too many gaps on test day. It simply speeds up your problem solving if the formulas are already automatic and lets you spend time where it counts the most.