If you query a publisher and they invite you to send in the first three chapters of your manuscript, please just do it! There have been several authors whose queries interested me so I asked for the first three chapters, only to have the authors balk, begging to send chapters from the middle of their book because the first three chapters aren't "representative of their work"! Well I've got news for you, if you haven't engaged the reader within the first three chapters then you have colossally failed as an author. When people are browsing for a book to buy, they don't crack open the middle of the book to check it out, they start reading the first chapter, and if it doesn't draw them in pretty quickly, they put it right back on the shelf. Whenever an author isn't happy with sending me only the first three chapters, I immediately see that as a red flag that they don't have faith in their own work.