A text has an introduction, a body, and a ...?

The word you're looking for is conclusion.

An alternative to conclusion could be a summary, if it comes to wrap up the piece.

If it contains ancillary information, It could also be a postscript or an appendix.

For a piece of fiction, I would call it an epilogue.

Could you be more specific about what is in this section? It's entirely possible that there isn't an all-encompassing word for it, but once we know what you have in your hands, we could find the right term.

Frankly I think "conclusion" is usually the best word, even if it really isn't a conclusion in the sense of being the logical result of a long argument or series of events. If you said, "And in the conclusion of the book, we learn that everything that came before was irrelevant," I doubt anyone would protest, "But then it's not really a 'conclusion', is it?" unless they really wanted to be argumentative.

Other possible words are:

denoument: A "wrapping up" section of a story, following the real climax and conclusion. Like, if you have a crime story, you might think of the chapter where the detective solves the crime and the villain is caught as the "conclusion". If there's then a final chapter about how the victim tries to put his life back together, or where the detective and the heroine have a romantic encounter, or whatever, that is often called a "denoument".

postlude: Indicates something at the end with no particular "role". It's ofen used for a denoument, but not necessarily.

appendix: Material that follows the main narrative. "Appendix" is often used for reference material, like a list of the main people in history book, or detailed calculations or statistics in a technical book.

end, or ending: Simple and general.

I guess you could make a technical case that an "introduction" is usually considered to be something separate from the main text. Often an introduction is written by someone other than the primary author. So in that sense the analogue would be something at the end that is also not part of the main text, which brings us back to "appendix" and "addendum".