Show content after the first and second paragraph

My way to do this (see update below):

function addParagraphs($content) {
    // you can add as many as you want:
    $additions = array(
        '<p>After 1st paragraph</p>',
        '<p>After 2nd paragraph</p>'
    );

    $content = get_the_content();

    $output=""; // define variable to avoid PHP warnings

    $parts = explode("</p>", $content);

    $count = count($parts); // call count() only once, it's faster

    for($i=0; $i<$count; $i++) {
        $output .= $parts[$i] . '</p>' . $additions[$i]; // non-existent additions does not concatenate
    }
    return $output;

}
add_filter('the_content','addParagraphs');

Answer is updated according to subsequent comments:

$paragraphAfter[1] = '<div>AFTER FIRST</div>'; //display after the first paragraph
$paragraphAfter[3] = '<div>AFTER THIRD</div>'; //display after the third paragraph
$paragraphAfter[5] = '<div>AFTER FIFtH</div>'; //display after the fifth paragraph

$content = apply_filters( 'the_content', get_the_content() );
$content = explode("</p>", $content);
$count = count($content);
for ($i = 0; $i < $count; $i++ ) {
    if ( array_key_exists($i, $paragraphAfter) ) {
        echo $paragraphAfter[$i];
    }
    echo $content[$i] . "</p>";
}