Wordpress: wrap each 3 posts into a div.

                    <?php query_posts('post_type=post'); ?>
                        <?php $variable=0;?>
                        <div class="carousel-item active">
                            <div class="service_row row">
                            <?php while ( have_posts() ) : the_post(); ?>
                            <?php if(($variable+1)<5){ ?>
                                <div class="col-lg-3 col-md-6">
                                    <div class="service_row__col ">
                                        <div class="pimage">
                                            <?php if ( has_post_thumbnail() ) { the_post_thumbnail(); } ?>
                                        </div>
                                        <h2 class="service_row__title"><?php the_title(); ?></h2>
                                    </div>
                                </div>
                            <?php $variable+=1; ?>
                            <?php }else{ ?>
                            <?php $variable=1; ?>
                            </div>
                        </div>
                        <div class="carousel-item">
                            <div class="service_row row">
                                <div class="col-lg-3 col-md-6">
                                    <div class="service_row__col ">
                                        <div class="pimage">
                                            <?php if ( has_post_thumbnail() ) { the_post_thumbnail(); } ?>
                                        </div>
                                        <h2 class="service_row__title"><?php the_title(); ?></h2>
                                    </div>
                                </div>
                            <?php }?>
                            <?php endwhile; ?>
                            </div>
                        </div>
                        <?php wp_reset_query(); ?>