The third option – a master CSS file organised into sections – is definitely the best option for maintenance and performance. While it may seem better to use a
STYLE element on individual pages, it is a false economy. In isolation, it is quicker (at least, in the short-term), but
STYLE elements are not individually cached, and must be reloaded with the HTML every time.
Also, I think that it is easier to manage your CSS when it's in one external file (or even a group of smaller, modular files), than it is to go rooting through your file system, hunting for the odd STYLE element here and inline
style rule there. Personally, it helps me to create a more consistent look and feel across a website.