This is probably my third favorite Beatles's song.
If you are looking for Koichi, he's reading Harry Potter on the third toilet from the left.
Did you know that Japan has the third highest number of inhabited islands in the world, after Indonesia and the Philippines?
Remember 一番目? This is just like that, but with three, making it the third thing or simply the third.
Use your knowledge of the reading for 一番目 to remember the reading for this word as well.