According to Cambridge Dictionary of English Language,
People is always used to refer to groups of human beings or humans in general.
"I saw three people standing on the corner."
Persons is a very formal word. We only use it in rather legalistic contexts.
"Any person or persons found in possession of illegal substances will be prosecuted".
Person is used in singular to refer to any human being (a man, a woman or a child)
"Joel is such a nice person."
