Multiple Custom Post type with intervals

This sounds very tricky. I would try using the Modulus operator to trigger a testimonial query after every 3rd slide.

Thanks graphicgeek. I tried your suggestion using modulus operator and here goes my query below

<?php
   global $post;
    $args = array(
	 'orderby' => 'menu_order',
	 'order' => 'ASC',
	 'showposts' => 15,
     'post_type' => array( 'slideimages' )
     );
 ?>

     <div id="slider">

<ul>

<?php
	$intervalpost = new WP_Query($args);
	$count=1;

	while ( $intervalpost->have_posts() ) : $intervalpost->the_post(); global $post; ?>

        <li style="color: #fff;"><?php the_content();?></li>
		<?php
		$count++;
        if ($count % 5 == 0 || $count % 9 == 0 || $count % 13 == 0){     

		 $argsnew = array(
		 'orderby' => 'menu_order',
		 'order' => 'ASC',
		 'posts_per_page' => 1,
		 'offset' => 0,
		 'post_type' => array( 'testi' )
		 ); 

		 if ($count % 5 == 0 || $count % 9 == 0 || $count % 13 == 0){
		 }

		 $the_query = new WP_Query( $argsnew );
		while ( $the_query->have_posts() ) : $the_query->the_post(); ?>

              <li style="color: #fff;"><?php the_title();?> <br /> <?php the_content();?></li>

			 <?php endwhile;

      // Reset Second Loop Post Data
      wp_reset_postdata(); 

        }?>  

		<?php endwhile;
	echo '</ul>';
	echo '</div>';
	 ?>

Now my problem is that, the result goes this way.

slide 1 -> slide 2 -> slide 3 -> testimonial 1 ->
slide 1 -> slide 2 -> slide 3 -> testimonial 1 ->
slide 1 -> slide 2 -> slide 3 -> testimonial 1 ->

Try using a second counter to count the number of times the testimonial query has been run, something like $testimonialcount, and then use this for $argsnew:

$argsnew = array(
		 'orderby' => 'menu_order',
		 'order' => 'ASC',
		 'posts_per_page' => 1,
		 'offset' => $testimonialcount,
		 'post_type' => array( 'testi' )
		 );

Also, for your if statement, you may be able to simplify it to if($count % 3 == 0) This would return true when the count is 0, 3, 6, 9, etc

Hope that helps.