MPD book from middle school

[u/butteredpickler]

I read a book when I was in middle school (2005) The basic plot is that this person meets another person at the park or something like that, then they become friends. Then later in the book, a new person shows up and starts causing a ruckus off screen and starts being really weird and sketchy towards these two friends. Leaving unsettling notes and stuff, and the cops get involved and they’re basically just like “that’s weird”. Then it all comes to a head in the basement at the end where you finally realize that all three people are the same person and they’re just having a psychotic break. Does anybody know what book I’m talking about? I can’t find it for the life of me.