Create a custom WP_Query, and loop through the results, making sure to reset the query afterward

secondary-wp_query.php

<?php

/**
 * create a custom query, using specific arguments, making sure to reset
 * the query afterward.
 */
function create_secondary_query() {

    // for list of possible arguments, @see https://developer.wordpress.org/reference/classes/wp_query/#parameters
    $args = array(
        'posts_per_page' => 3 , // how many posts to return?
    );

    ob_start();

    $secondary_query = new WP_Query( $args );

    if ( $secondary_query->have_posts() ):

        while ( $secondary_query->have_posts() ): 

            $secondary_query->the_post(); ?>

            <?php    
            // do something with each post here ?>

            <?php
            // end while $secondary_query->have_posts():
        endwhile;

        // end if $secondary_query->have_posts():
    endif;

    return ob_get_clean();

    wp_reset_query();

}