Add Smart Slider 3 to Twenty-Seventeen theme

To replace the theme's default image with a slider, you should open this file: \wp-content\themes\twentyseventeen\template-parts\header\header-image.php
And locate this code:

<div>
    <?php the_custom_header_markup(); ?>
</div>

Which creates the image. You should replace the above PHP code with your slider's PHP code. E.g::

<div>
    <?php echo do_shortcode('[smartslider3 slider=1]'); ?>
</div>

And your whole theme file should look like this:

<?php
/**
 * Displays header media
 *
 * @package WordPress
 * @subpackage Twenty_Seventeen
 * @since 1.0
 * @version 1.0
 */

?>
<div class="custom-header">

		<div class="custom-header-media">
			<?php echo do_shortcode('[smartslider3 slider=1]'); ?>
		</div>

	<?php get_template_part( 'template-parts/header/site', 'branding' ); ?>

</div><!-- .custom-header -->