This may be a really stupid question, but when a publisher or agent asks for three chapters, do they mean the first three chapters? Or three random chapters? If my book has a prologue, is that considered the first chapter? Or do I send the prologue plus three chapters?
You send the first three chapters. Always. [Unless specifically instructed otherwise by the agent or publisher.]
If you have a prologue and it's absolutely critical to understanding what is happening in the first three chapters, then call it chapter 1 and send the next two chapters (renumbered as 2 and 3). Most of the time, however, you do not send the prologue.
This is only a stupid question if you ask me after you've sent in three random chapters.