As an IMO gold medalist, I can say that understanding all the theory in the world won't guarantee a good score in the IMO. IMO is not just centred around theory that you can learn in books; it involves ingenuity and intuition that often cannot be taught. Even then, it took hundreds of hours of comprehensive study over many, many topics, much more diverse than the three you've listed, which don't even touch on geometry or number theory, and barely go into algebra. This renders three quarters of the IMO inaccessible if that theory is all you know. However, personally, I found that doing a large number of problems, and learning the theory for them as I went, was much more useful than studying theory then trying to apply that theory to problems.
In the end, it’s important to enjoy the maths you do. IMO isn’t like a school exam where you study for hours being bored - it’s ultimately a journey of learning and improvement, and it’ll be worth it no matter how well you do in competitions.