Nikon D3 100 SLR camera can't focus automatically. What should I do?

Nikon D3 100 camera is an entry-level model. The camera itself does not have a focusing motor, and the lens needs an ultrasonic focusing motor to achieve automatic focusing. I don't know which lens you are using.

If your lens is equipped with a focus motor, but it can't focus, it is mostly because of poor interface contact. Just take off the lens and reinstall it. When the battery power is insufficient, it will not be able to focus automatically.