Set 3 iframes in a row

The problem is that you are using the_excerpt(). This function prints out the excerpt immediately, it doesn’t return a string.

From the core:

/**
 * Display the post excerpt.
 *
 * @since 0.71
 */
function the_excerpt() {

    /**
     * Filters the displayed post excerpt.
     *
     * @since 0.71
     *
     * @see get_the_excerpt()
     *
     * @param string $post_excerpt The post excerpt.
     */
    echo apply_filters( 'the_excerpt', get_the_excerpt() );
}

So you need to circumvent the echo by using apply_filters( 'the_excerpt', get_the_excerpt() ) instead of the_excerpt(), then you get the string you need, and the iframes should be where they belong.