有两个步骤可以实现此功能：

在主循环中添加一个计数器，以便检查输出的帖子数量。

如果计数器小于3，请再次运行查询并输出。

因此，对于步骤1，在循环之前设置一个计数器，并在每次循环运行时递增，如下所示：

<?php
// create your counter variable
$counter = 0;
if (have_posts()) :
    while (have_posts()) : the_post();
    // add 1 to your counter, as 1 post was just output
    $counter++;
    // (display the posts)
    endwhile;
endif; ?>

<?php
// if counter is less than 3, run a separate query
if($counter < 3) {
    // get the ID of the current Tag
    $tag_id = get_queried_object()->term_id;
    // set up query arguments
    $args = array(
        // only pull Posts
        \'post_type\' => \'post\',
        // don\'t pull Posts that have the current Tag - prevents duplicates
        \'tag__not_in\' => array("$tag_id"),
        // set how many Posts to grab
        \'posts_per_page\' => 3
    );
    $extraPosts = new WP_Query($args);
    if($extraPosts->have_posts()):
        while($extraPosts->have_posts()) : $extraPosts->the_post();
        // display the posts)
        endwhile;
    endif;
} ?>

$numberToGet = 3 - $counter;