Just in case someone is still looking for the answer,
let the browser take care of that. Try this:
display: table
on the container element.display: table-cell
on the child elements.The browser will evenly divide it whether you have 3 or 10 columns.
EDIT
the container element should also have: table-layout: fixed
otherwise the browser will determine the width of each element (most of the time not that bad).