Which body system contains the cerebrum, cerebellum, and medulla oblongata?

Nervous

Nervous system.

Nervous system can be divided into central nervous system(CNS) and peripheral nervous system(PNS).
 
CNS-consists of brain and spinal cord.

Cerebrum, cerebellum and medulla oblongata made up of the brain, thus are part of brain and central  nervous system.

going to have to use your head here...

The cerebrum or telencephalon, together with the diencephalon, constitute the forebrain. It is the most anterior or, especially in humans, most superior region of the vertebrate central nervous system. (grey matter on top)

The cerebellum (Latin for little brain) is a region of the brain that plays an important role in motor control. (red bit at back of neck)

The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla. The medulla contains the cardiac, respiratory, vomiting and vasomotor centers and deals with autonomic functions, such as breathing, heart rate and blood pressure.

The Nervous system!

The Cerebrum and cerebellum can be found in the brain, and the Medulla oblangata is the site of regulation in the body!:)