1.perhaps,maybe是副词,用来修饰句子,用在句子开头表示“或许”的意思,比如 Perhaps he will forgive you,或者Maybe this is the best choice.
2.may be则是情态动词+be动词,用作句子的谓语,表示“或许是”,比如 He may be the one who broke the window.
3.possible则是个形容词,象一般形容词一样,可以修饰名词,如possible solution,或者用在be动词后作状语,如 Everything is possible