Is this not a case for using {switch="||do-thing"}?

{switch='<div class="row">||'}
        {cf_about_us_biographies}
        <div class="four columns">
            <img src="{cf_about_us_bio_photo}">
            <ul class="accordion">
                <li>
                    <div class="title">
                        <h5>{cf_about_us_bio_name}<br><span>{cf_about_us_bio_title}</span></h5>
                    </div>
                    <div class="content">
                        {cf_about_us_bio_biog}
                    </div>
                </li>
            </ul>
        </div>
        {/cf_about_us_biographies}
 {switch='||</div>'}

The issue there though is if you have a number of items that isn't divisible by 3. From memory there's a useful plugin for handling this but I can't recall its name off the top of my head.

Try looking here Output matrix rows in sets of two

Is this not a case for using {switch="||do-thing"}?

{switch='<div class="row">||'}
        {cf_about_us_biographies}
        <div class="four columns">
            <img src="{cf_about_us_bio_photo}">
            <ul class="accordion">
                <li>
                    <div class="title">
                        <h5>{cf_about_us_bio_name}<br><span>{cf_about_us_bio_title}</span></h5>
                    </div>
                    <div class="content">
                        {cf_about_us_bio_biog}
                    </div>
                </li>
            </ul>
        </div>
        {/cf_about_us_biographies}
 {switch='||</div>'}

The issue there though is if you have a number of items that isn't divisible by 3. From memory there's a useful plugin for handling this but I can't recall its name off the top of my head.

Try looking here Output matrix rows in sets of two