A bit esoteric even for a twelve-year-old.
Increasing and decreasing refers to the content of each part of the tricolon. So an increasing tricolon would have the final statement more important than the first: 'I came, I saw, I conquered'. Decreasing would be the reverse.
I'm sure you've looked at these: https://en.wikipedia.org/wiki/Isocolon
http://changingminds.org/techniques/...h/tricolon.htm