Is stage 3 ckd serious

@victoriasitko Here is an article from DaVita.com, about phosphorous: https://www.davita.com/diet-nutrition/articles/advice/top-low-phosphorus-foods-for-a-healthy-kidney-diet

You also asked if stage 3 is serious. There are several factors. There are two subcategories, "a" and "b" depending on your eGFR number. And, remember it is best to determine the underlying cause of the decline in kidney function, be it another health condition, genetics, medication reaction, lifestyle. Then you can best tailor the situation that fits you.
Ginger