I don’t quite agree with this. It depends on how often you change your mind about the third field, and what your standard for “quite good” is. If you put a few years of effort into a field and get to a high level of competence and productivity, that skill may stick with you for a long time, even if you fall out of practice.

For example, let’s say that you play piano, and you push yourself to a new level of skill and learn a few more advanced pieces. The decay rate for that skill is quite long, and with a very minimal level of work you can keep those pieces fresh for years or decades.