{# Assuming a page object with one or more instances of a content block: #}
{% for block in page.blocks %}
    {# Start a new row every third block. #}
    {% cycle '<div class="row">' "" "" %}
        <div class="col-md-4">
            {{ block }}
        </div>
    {# End every third row. #}
    {% cycle "" "" '</div>' %}
{% endfor %}

{# The row div will need to be closed if the total blocks is no divisible by 3. #}
{% if not page.blocks|length|divisibleby:"3" %}</div>{% endif %}