Generally it's a good idea to use the first three chapters. I once heard some good advice: If your first chapters aren't your strongest, then make them stronger. Otherwise, why are they there?
I've found that to be very true. So if you're hesitating to send your first three chapters, try to understand what the problem is. Do they really belong there? Could they be cut? Is your story beginning in the right place?
Those first three chapters are critical, and the first sign that that something is off is that you'd rather send others.