For eg. 3 consecutives banners/skyscrapers, one on top of the other. Or 2/3 square link ads units, placed beside each other
i use consecutive sky scrappers on my blog sites because the page is usually pretty long from the posts. example http://bobdylannews.blogspot.com
i dont think there is any problem in that.. but you can have different combinations which can convert better for you so i wont recommend it.