Chapters
One thing to remember about chapters, and why having a full manuscript requested still does not put the odds in a writer's favor.
I've read many a partial that was very well-written. The full manuscript also proved to be very well-written. But what the full manuscript often does not turn out to be is 1. Marketable. 2. A good story start to finish. 3. Original. 4. Logical.
All the first three chapters really tells an agent or editor is whether you write dialogue well, sometimes whether you know what goes into a good character, etc. Butthose first three chapters say nothing about how well you can write a novel. It really doesn't have anyting at all to do with teh writing going downhill after the first three chapters. That can be fixed with a minimum of fuss. If you can write the firstthree chapters well, I can be pretty sure you can fix any writing problems in later chapters.
But writing well does not a marketable novel make. A sustained, entertaining story, good characters who develop as they should, good structure, good pace, good rhythm, good arc, tension in the right places, conflict of the right kind at the rights places, mood and tone, a plot that holds together from beginning to end, etc., etc., are what make a novel marketable.
Three good chapters can give an agent or editor hope that the full novel will have all these things, and this is why a full manuscript is requested. But in all truth, only one or two percent of requested novels actually do prove to have all these things done well enough to make the cut.
How well the first three chapters are write usually have very little bearing on how good, or how marketable, the overall novel is. While a well-written partial is important, without it few will ask to see the complete manuscript, that partial, no matter how well it may be written, in no way means the complete novel will have a story, characters, plot, conflict, tension, on and on and on, that can hold my interest all teh way through.