Three sentences is fine if that's all it takes to provide a snapshot of your book. But sometimes you need more, and two or even three paragraphs is perfectly acceptable. Nor do I see any reason to avoid character names on principle. In fact, while leaving your characters nameless might work in some circumstances, this seems to me like fairly bad advice in many cases. In some genres, such as fantasy, names are an important part of the setting.
Everyone you ask about query letters will tell you something different. I think there are really just a few basic rules: include genre and word count, make your plot snapshot as interesting and compelling as you can, keep it to a single page, mention your credits ONLY if they're relevant. And remember that it's a business document. Other than that, there's quite a lot of leeway. Your job in the query isn't to stick to someone else's idea of a good query; it's to intrigue an agent or editor so much that they'll want to read more.
- Victoria