Empty the Jetpack Sharing Label input; localize a script to feed it a custom value, wrapped in a p tag.

custom-jetpack-share.php

<?php 
/**
 * If the Jetpack Sharing label setting has a value, remove it.
 * If that field is empty, Jetpack Sharing will not output a set of H3 tags with the label.
 *
 */
function sharing_label_clear() {

	if ( class_exists( 'Sharing_Service' ) ) :
		
		// read a field from a theme options page
		$sharelabel = get_option( 'sharing-options' );
		
		// $label comes from the Sharing_Service class
		if ( $label != NULL ) :
			$sharelabel['global']['sharing_label'] = '';
			update_option( 'sharing-options', $sharelabel );
		endif;

	endif;

}


/**
 * Gets the ACF options field, sharing_label and sends it to a javascript
 *
 */
function sharing_label_script() {

	if ( class_exists( 'Sharing_Service' ) && !is_single() ) :

		$new_label = get_field( 'sharing_label','option' );
		$js_loc = get_stylesheet_directory_uri().'/lib/js/';
		wp_enqueue_script( 'sharing-set', $js_loc. 'sharing-set.js', array('jquery'), '0.1', false );
		wp_localize_script( 'sharing-set', 'ShareLabel', array( 'text' => '<p class="sd-title">'. $new_label .'</p>' ) );

	endif;

}