First things first.... extra layers mean bigger dumps. Depending on the condition/age of the roof will depend on how much extra time it will take to remove. Also depending on the age/condition there may be some bad wood under there, which can get costly.
IMHO second and third layers are on a sliding scale based on the factors above.
I have been on some two layer roofs that I didn't have to pick up a tear off shovel to remove and they were gone in a few hours, then I have been on two layers that left my elbows covered in ben-gay at the end of the work day knowing I had to go back for more tomorrow.
Not sure if you pay your labor by the hour or by the square, but if your paying by the hour sharpen your pencil.