<?php

    $fields =  array(
        'author' =>
          '<div class="col-md-4 comment-form-author">' .
          '<input placeholder="'.__( 'Name', '_lp' ).' '.( $req ? ' ( Required )' : '' ) .'" id="author" name="author" type="text" value="' . esc_attr( $commenter['comment_author'] ) .
          '" size="30"' . $aria_req . ' /></div>',

        'email' =>
          '<div class="col-md-4 comment-form-email">'.
          '<input placeholder="'.__( 'Email', '_lp' ).' '.( $req ? ' ( Required )' : '' ) .'" id="email" name="email" type="text" value="' . esc_attr(  $commenter['comment_author_email'] ) .
          '" size="30"' . $aria_req . ' /></div>',

        'url' =>
          '<div class="col-md-4 comment-form-url">' .
          '<input placeholder="'.__( 'Website', '_lp' ).' '.( $req ? ' ( Required )' : '' ) .'" id="url" name="url" type="text" value="' . esc_attr( $commenter['comment_author_url'] ) .
          '" size="30" /></div>',
        );
    ?>

//for the comment wrapping functions - ensures HTML does not break.
$comment_open_div = 0;

/**
 * Creates an opening div for a bootstrap row.
 * @global int $comment_open_div
 */
function _lp_before_comment_fields(){
    global $comment_open_div;
    $comment_open_div = 1;
    echo '<div class="row">';
}
/**
 * Creates a closing div for a bootstrap row.
 * @global int $comment_open_div
 * @return type
 */
function _lp_after_comment_fields(){
    global $comment_open_div;
    if($comment_open_div == 0)
        return;
    echo '</div>';
}

add_action('comment_form_before_fields', '_lp_before_comment_fields');
add_action('comment_form_after_fields', '_lp_after_comment_fields');

<div class="row">
<!--the fields and stuff-->
</div>

function wpse172052_comment_form_before_fields() {
    echo '<div class="row">';
}
add_action('comment_form_before_fields', 'wpse172052_comment_form_before_fields');

function wpse172052_comment_form_after_fields() {
    echo '</div>';
}
add_action('comment_form_after_fields', 'wpse172052_comment_form_after_fields');