What are 3 volt dc batteries used in?

Assuming you are misusing the term 'battery' when you should really write 'cell', 3V Lithium cells (button cells) are often used in wristwatches. Some computers and maybe smartphones use them to power their realtime clocks. It is possible that some hearing aids use them also, but many use Silver cells, which have a lower voltage.

3V is a convenient voltage that allows modern electronics, both digital and analog, to work in very low-power devices.