First of all u need to understand who is "person" as per IT act, 1956: Person includes, 1) Individual 2) Hindu Undivided Family 3) Partnership Firm 4) Company 5) Association of person/Body of Individuals, whether incorporated or not 6) Local Authority 7) Any other Artifical Juridical Person
Now the difference among these three is: 1) Association of "Person", the word in inverted commas signifies that the different categories of the term person can become member of association. e.g., a Company, partnership firm and local authority may form association and it will be called AOI and will be assessed as such under IT act apart from there individual identity.
2) Body of "Individual", the word in inverted commas is just a category of the term Person, and hence only individuals may come forward to form a body and it will be called BOI and will be assessed as such under IT act apart from individuals personal identity.
3) Firm is assessed as an independent entity under IT Act.