bloginfo(‘template_directory’) img src

You cannot use bloginfo() while your are outputting using echo because bloginfo it self also out puts string using echo. Below will work for you, you also have extra double quote which i have removed….

<?php   

        $attch_id_1 = pn_get_attachment_id_from_url(get_post_meta($post->ID, 'img1', true));
        $image_attributes_1 = wp_get_attachment_image_src( $attch_id_1, 'full'); 

        $attch_id_2 = pn_get_attachment_id_from_url(get_post_meta($post->ID, 'img2', true));
        $image_attributes_2 = wp_get_attachment_image_src( $attch_id_2, 'full');

        $attch_id_3 = pn_get_attachment_id_from_url(get_post_meta($post->ID, 'img3', true));
        $image_attributes_3 = wp_get_attachment_image_src( $attch_id_3, 'full');

        echo '<img src="'.get_bloginfo('template_directory')."https://wordpress.stackexchange.com/images/img-BG.png" data-original="'.$image_attributes_1[0].'">';

        echo '<img src="http://localhost/wordpress-cd/wp-content/themes/cd/images/img-BG.png" data-original="'.$image_attributes_2[0].'">';

        echo '<img src="http://localhost/wordpress-cd/wp-content/themes/cd/images/img-BG.png" data-original="'.$image_attributes_3[0].'">';

    ?>