Best way to represent 1/3rd of 100% in CSS?

Are you making any allowance for margins? You could go 30% per column with 5% margin either side of the center column.

Quick example

Now that calc is widely supported among modern browsers, you can use:

#myDiv {
    width: calc(100% / 3);
}

Or you can use this as a fallback if your browser wouldn't support it:

#myDivWithFallback {
    width: 33.33%;
    width: calc(100% / 3);
}