How to automatically load an image inside the post, since it is present in uploads folder

You should check if those files exist before you output that div. You can clean up your markup and get rid of that onerror call by wrapping each image div in a file_exists check. Use a ternary to check if the file exists and set the $image variable if it is. Like so:

$image1 = "/nomes-unisex/" . $post->post_name . ".jpg";
$image2 = "/nomes-femininos/" . $post->post_name . ".jpg";
$image3 = "/nomes-masculinos/" . $post->post_name . ".jpg";
$uploadDir = wp_get_upload_dir();

$image1 = file_exists($uploadDir['basedir'] . $image1) ? $uploadDir['baseurl'] . $image1 : '';
$image2 = file_exists($uploadDir['basedir'] . $image2) ? $uploadDir['baseurl'] . $image2 : '';
$image3 = file_exists($uploadDir['basedir'] . $image3) ? $uploadDir['baseurl'] . $image3 : '';
?>
<div class="acf_imgs-centro1">
<?php if ($image1 || $image2 || $image3) {
    ?>
    <h2 class="imgs_h2">Compartilhe nas redes sociais</h2>
    <?php if ($image1) { ?>
        <div class="img-neutra">
            <img alt="Significado de <?php the_title(); ?>" src="https://wordpress.stackexchange.com/questions/330251/<?php echo $image1; ?>">
        </div>
    <?php }
    if ($image2) { ?>
        <div class="img-fem">
            <img alt="Significado de <?php the_title(); ?>" src="<?php echo $image2; ?>">
        </div>
    <?php }
    if ($image3) { ?>
        <div class="img-masc">
            <img alt="Significado de <?php the_title(); ?>" src="<?php echo $image3; ?>">
        </div>
    <?php }
} ?>
</div>