The Bermudez Triangle by Maureen Johnson

'Their friendship went so far back, it bordered on the Biblical-in the beginning, there was Nina and Avery and Mel.' So says high school senior Nina Bermudez about herself and her two best friends, nicknamed 'The Bermudez Triangle' by a jealous wannabe back on Nina's eleventh birthday. But the threesome faces their first separation when Nina goes away the summer before their senior year. And in ten short weeks, everything changes. Nina returns home bursting with stories about Steve, the quirky yet adorable eco-warrior she fell for hard while away. But when she asks her best friends about their summer romances, an awkward silence follows. Nina soon learns the shocking truth when she sees Mel and Avery . . . kissing. Their friendship is rocked by what feels like the ultimate challenge.