How can I make B be a fixed 800px size while A and C fill the space, so they are all together touching? http://jsfiddle.net/8ufuV/
The easiest method is to use display: table-cell.
display: table-cell
See: http://jsfiddle.net/47d4f/
That solves your problem, and also gives you equal height columns when the content is different in each column - something that isn't otherwise simple to obtain.
Browser support: http://caniuse.com/css-table
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