Among Friends (Vintage Paperback) by Caroline B. Cooney

In this fast-paced novel about the bonds of friendship, Jenny, Emily, and Hillary, known as the Awesome Threesome, thought they'd be a threesome forever. But with Jennie's relentless ambition, Emily's jealousy, and Hillary's building resentment, it seems the Awesome Threesome is destined to be a thing of the past.

Assigned to write in diaries for English class, the three girls, along with three other classmates, find themselves admitting things so terrible that even they are shocked. All are left wondering: What price does a person have to pay for being successful? The pursuit of the truth is compellingly chronicled and the resolution satisfies.