This adds a parent wrapper div every 3 children using Liquid's modulo filter. This uses bulma CSS classes, but applies to any CSS framework

jekyll-modulo-columns.html

<!-- This loops through the paginated posts in Jekyll -->
{% for post in paginator.posts %}
    <!-- Modulo counter in liquid, will enclose div.columns every 3 posts starting at 0 -->
    {% assign mod = forloop.index0 | modulo:3 %}
    <!-- If the modulo counter is 0, then open a div with class columns -->
    {% if mod == 0 %}
    <div class="columns">
    {% endif %}
        <!-- This is the post div, it will be repeated 3 times -->
        <div class="column">
            <h1>
                <a href="{{ post.url }}">{{ post.title }}</a>
            </h1>
            <div>
                <span>Day {{ post.date | date: "%j" }}</span>
            </div>

            <div class="content">
                {{ post.content | strip_html }}
            </div>
        </div>
    <!-- Closes the columns parent, skips the first 2 post divs, and adds to every 3rd post afterward-->
    {% if mod == 2 %}
    </div>
    {% endif %}
{% endfor %}
</div>