Why is my three year old son always thirsty?

Thirsty: He certainly should be tested for diabetes mellitus. Diabetes insipidus can also be a remote cause. Sleeping with an open mouth secondary to enlarged tonsils is another cause. See your pediatrician for further help.

Many possibilities: Some kids just develop a pattern of carrying a cup everywhere and drinking as a habit. They pee a lot but sleep without getting up to drink and don't have to drink in other settings. Some have medical issues needing study. They can be of life altering importance. A trip to your doc with some urine studies can start the process of sorting this out.