Involve Free Roof Estimates 630 915 7663 Promar Roof. It is fine to get three layers of shingle codes it requires should check this is actually case with your nearby building permit people and factors to consider that your metal roof top, shingle roof, tile roof structure, flat roof, and cedar shake roof and all of those other structure have the strength to handle the extra weight.
http://www.youtube.com/watch?v=lhIEvZ5tEfI