These words have different meanings in different countries. As far as I know, the term "barrister" is only used in England, Wales, Northern Ireland, Ireland, some Australian states, and some other Commonwealth countries. It refers to a lawyer who specialises in courtroom advocacy and providing expert legal opinions. Barristers are generally not instructed directly by clients, but via other lawyers. The word "attorney" is not usually used in England to refer to a lawyer, though it has some specialist legal usages here. As far as I know, it is the common word for a practising lawyer in the USA. An "advocate" is a general term for someone who represents another person, usually orally. The word is used in some specialist situations to refer to some lawyers in England. It may be a term commonly used for lawyers in some other countries but I do not know for sure.

An advocate (AD-və-kit) is someone who supports a cause, like an advocate for outdoor recess. Advocate (AD-və-kate) is also a verb meaning to speak in favor of, so you can advocate for that outdoor recess by urging your school to play outside! The Brits and Canadians prefer to say barrister instead of "lawyer," but they mean the same thing. A barrister is a person who goes to court on behalf of the defense or the prosecution. An attorney is a lawyer. Attorneys sue people, defend people, and serve as experts on the law. When there's any kind of legal issue, attorneys will be involved. They're the legal experts with law degrees who are also known as "lawyers." You'll often find them in court, defending clients or trying to put criminals in jail. Attorneys work for businesses, schools, the government, and individuals. So if you get arrested, need to write a will, or have to hash out a binding contract, an attorney is the person to call.

In what country?