functions.php

<?php

/**
 * Change comment form textarea to use placeholder
 *
 * @since  1.0.0
 * @param  array $args
 * @return array
 */
function ea_comment_textarea_placeholder( $args ) {
	$args['comment_field']        = str_replace( 'textarea', 'textarea placeholder="comment"', $args['comment_field'] );
	return $args;
}
add_filter( 'comment_form_defaults', 'ea_comment_textarea_placeholder' );

/**
 * Comment Form Fields Placeholder
 *
 */
function be_comment_form_fields( $fields ) {
	foreach( $fields as &$field ) {
		$field = str_replace( 'id="author"', 'id="author" placeholder="name*"', $field );
		$field = str_replace( 'id="email"', 'id="email" placeholder="email*"', $field );
		$field = str_replace( 'id="url"', 'id="url" placeholder="website"', $field );
	}
	return $fields;
}
add_filter( 'comment_form_default_fields', 'be_comment_form_fields' );

style.css

.comment-respond label {
	display: none;
}

Wouldn't be better to add the .screen-reader-text class to the labels instead of display: none for accessibility?

Another use of .screen-reader-text is to hide labels in forms. Search forms are a common place where designers use placeholders and image buttons to convey the purpose of the form. Adding a label that’s available to screen readers make the form usable for screen reader users, without altering the design.
-Hiding text for screen readers with WordPress Core