The epithalamus, perithalamus, thalamus and hypothalamus have in common that they are all part of the brain region known as the diencephalon. This structure is extremely important as it will be responsible for the collection, interpretation, and relying of information from the midbrain and other systems towards the cerebral cortex, and other parts of the brain, and then will be responsible also for collecting the responses from the higher brain regions towards the appropriate places.
However, each of these structures is different and has a different purpose.
The eipthalamus, which would be the outermost structure of the diencephalon, is the first region where information is relayed by the midbrain and the lymbic system. This structure is responsible also for the secretion of melatoning, which controls the circadian rythm, present in its most important portion, the pineal gland. It also controls the secretion of certain hormones of the pituitary gland, but it is not its biggest role.
Then comes the thalamus. This structure is also really important as it is also a centre of collection of information, especially that relayed by the midbrain, towards the cortex. Without the thalamus, there would be no connection between the cerebral cortex, the midbrain, and other parts of the body. Another major role of the thalamus is to regulate all regarding the process of sleep, wakefulness and alertness in the body
Finally, we have the hypothalamus, and its importance its enormous. This small portion of the diencephalon is responsible for the whole process of homeostasis, which means, it regulates all the processes that will bring equilibrium to the body. It also works as a bridge between the nervous and endocrine systems, as the hypothalamus controls the secretion of hormonal messengers from the pituitary gland towards other glands.
These are only a few of the differences between these portions of the diencephalon, without mentioning the anatomical and physiological differences between them.
