As others have said, 3 layers is not a good idea and it does not meet most local codes. On the other hand, older homes often do turn out to have 3 layers because someone went ahead and added a 3rd layer without doing the tear-off that should have been done first. If you had a home inspection done before the purchase, that definitely would have given you the info on the status of the roof. As far as trying to go back now and stick it to the seller, that's doubtful at this point. The fact that you were now able to discover the status of the roof by checking on your own means you could have done the same thing before you bought the property.

If you don't see any evidence of sagging, and the roof seems to be in good shape and is not leaking, you may not need to do anything. If you are getting close to needing a new roof anyway, it would probably make sense to go ahead and get estimates and have it done now before this coming winter.