Welcome to being a parent. Throughout their lives they will find things that they desire above all else. Your goal should be to figure out how to use that to teach them moderation, and 3 is about the right place to start in my experience.

The primary goal for me at that stage was to help them understand delayed gratification, or specifically that working on something not as fun can lead to something fun. The secondary goal turned out to be to find other things that are just as fun. This latter goal was abetted by reading stories like "Bread and Jam for Frances" in which our heroine learns that while Bread and Jam is tasty, it isn't all there is in the world.

I did not realize it at the time but the secondary goal of finding things equally fun becomes really useful as they get older. To know from experience that no matter how fun this thing you are doing is, there are other things that are fun out there waiting to be discovered, helped mitigate some obsessing on the one fun thing. Reading/books are great for that since they come in many shapes and topics and you can learn a ton from them.