I have a building with 3 roofs on it, the top layer is starting to curl, etc, due to its age. My question is if having 3 layers of shingles will extend the useful life of the roof? I know this is vague but I hope you see where I'm going with this.
No. Layers of shingles on top of each other shorten the life of the roof due to the newest layer sitting on an uneven lower surfaces.
The best life will come from removing all the old layers down to roof boards or sheathing and then laying down the new roof on a well repaired flat surface.
Multiple layers of shingles happen because folks want to avoid extra removal work or to save the cost of removal labor and disposal costs. There is basically no truth to claims of benefits of stacking shingle layers.
As Michael Karas said, this happened because the previous owner was possibly lazy or too cheap to afford labor and disposal... or they may have been physically impared and living on a budget.
Adding an extra layer of shingles when the first layer has met it's life expectancy can buy a few more years... maybe 5 - 10 years but the life expectancy of second layer is less than it could be (10-30) years. The third layer is a waste- only lasts a few years (maybe).
Most importantly, the extra layers adds a lot of weight (pushing the load limit for the roof). This is kind of dangerous as it might cause the roof to collapse, (especially if you get much snow).
The rule of thumb is three layers maximum. After that you pull them all off and start over.
Half a century is about time to get a good look at the sheathing, and if any of it is bad, a good time to inspect the joists as well. I'm not saying you need a structural engineer, but you'll want someone on the squad who knows when to say, "This has all got to go." Be it just the roofing, the sheathing, or on to a few new joists.
Where you're going with this, is completely new roof(ing), assuming it leaks. The old shingles aren't helping; they all have nails through them from the latest install.
Beyond three layers, weight becomes a pressing concern.
in my time as a project manager for a construction company in TX and CA I learned that 3 is the maximum number of layers (legally) allowed anyway. If you're near a coast with hurricanes or in tornado areas, 2 layers is the max due to length of fasteners required to hold into the wood. Time to take them down, inspect the framing / sheathing and start fresh.