It's not American slang; it's internet slang, I guess? (The following is an explanation for anyone who both reads MoR and these discussion threads but isn't familiar with fanfiction in general.)
"Ship" is a term of art in fan communities deriving from "relationship" that indicates you think two fictional characters in some fictional universe should be together, e.g. "I ship Harry and Hermione" means "I think Harry and Hermione should be together." A substantial amount of fanfiction is centered on shipping, e.g. you might write fanfiction where Harry and Hermione get together explicitly because you are dissatisfied with the fact that it didn't happen in canon.
"Shipping wars" are a kind of conflict that can occur in fan communities between people who ship different couples involving the same fictional characters, e.g. Harry/Hermione vs. Ron/Hermione.
"OT3" is short for "One True Threesome"; it derives from "OTP," which is short for "One True Pairing" and refers to a couple that you ship very strongly, and I guess it means a threesome that you ship very strongly, e.g. Harry/Hermione/Ron. I suppose an OT3 is one way to resolve a shipping war...
Which is especially appropriate, given the other things that happen in this chapter.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 87. The previous thread has passed 500 comments.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: