Metal Roof with 3 layers???

From a practical standpoint, you are not alone. The low weight of metal roofing has allowed applications like this in other places as well. Your original wood shingles probably are in top of spaced boards so they are serving as the solid decking to support the other roofs. While I would not advise this sort of installation (primarily for the exact reasons you're facing today), to my way of thinking, the possible folks to raise issues with this are home inspectors, mortgage companies, insurance companies, and building inspectors. I do not know whether you're in an area where building codes with two roof maximum rules are enforced but it's likely you'd be "grandfathered" regardless. I also do not know whether you're in an area where insurance companies aggressively inspect roofs. Those would be primarily coastal areas. If you want to know whether roof replacement might be required soon (and my hunch is that it won't), you could go to all four of these groups, explain the situation, and ask them. The mortgage company is likely to raise a fuss only if the inspector raises a fuss. For your own peace of mind, you could have a structural engineer evaluate the roof but my guess is, unless there are active leaks and rot, it's probably just fine. When it was installed, the aluminum roof probably added less weight than the asphalt shingles had lost in terms of worn-off granules and evaporated oils.