All are trained extensively and professionally. Whilst there is some overlap in the functions of all three, there are important differences.
Broadly, Psychology is the science of addressing behaviour and mental health through thinking, reflection and changing behaviour. Psychotherapy is the process concerned with interaction, focusing upon a felt condition, or behaviour, in order to explore the underlying feelings, experience and thoughts which drive that condition, in past and present settings. A Psychiatrist has trained in medicine, followed by specialisation in the area of mental health. The Psychiatrist is likely to use drugs as therapy to deal with behaviour and mental conditions. Psychologists and Psychotherapists do not prescribe drugs, and instead are trained to enlist the thinking and reflective processes to address behaviour and mental conditions.