Upcoming CSS3 features (some browsers support some, but not others).

    <section id="box">
        <div class="left">...</div>
        <div class="center">...</div>
        <div class="right">...</div>
    </section>

- Template layout module (W3C)

    #box { display: '  a   b  c'
                     200px * 200px;
           width: 100%; }
    #box .left { position: a; }
    #box .center { position: b; }
    #box .right { position: c; }

- Flexible box model (Mozilla)

    #box { display: box; box-orient: horizontal; width: 100%; }
    #box .left, #box .right { width: 200px; }
    #box .center { box-flex: 1; }

- Grid positioning module (Microsoft)

    #box { columns: 3; grid-columns: 200px * 200px; }
    #box.left, #box.right { width: 200px; }

Disclaimer: I didn't test anything, just wrote the code from memory, peeking at random on-subject Google results. It is possible that I forgot, misspelled or misunderstood something.