nested divs, classes for a grid in loop [duplicate]

You don’t have to run through The Loop in one go.

$args = array(
    'posts_per_page' => 3,
    'category__in' => array(
        4,
        5,
    ),
);

$querytest = new WP_Query($args);

if ($querytest->have_posts()) {
    // this is the first post, with its markup
    $querytest->the_post();
    ?>
    <div class="width-1-2">
        <?php get_template_part('content', 'front', get_post_format());
    </div>
    <div class="width-1-2">
        <div class="row">
            <?php
            // here com the second and third post, wrapped in their additional markup
            for ($i = 0; $i < 2; ++$i)
                if ($querytest->have_posts()) {
                    $querytest->the_post();
                    ?>
                    <div class="width-1-2">
                        <?php get_template_part('content', 'front', get_post_format());
                    </div>
                    <?php
                }
            ?>
        </div>
    </div>  
    <?php
} else {
    // no posts found
}

I hope this gives you an idea and gets you started customizing this to suit your needs.