All three disciplines share interest in the workings of the mind and, if trained, can provide psychotherapy.
In general, Psychiatrists obtain a medical degree and therefore focus on the biological underpinnings of mental illness. Because they have a medical degree, psychiatrists can prescribe medication.
Psychologists earn a graduate level degree that focuses on all aspects of human behavior and personality structure, both normative and nonnormative.
Social Workers earn a graduate level degree with an emphasis on societal factors impacting emotional well-being and the need to connect people with the community and support services available.
Any of these three disciplines can choose to specialize their training in psychotherapy.