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3"/15' is just under 1/4" per foot. Do you really think you will even notice that? I wouldn't shim it.
Thanks to all for replies...some gave me hope, others made me cringe. However, after going out to the patio today to actually get a level line out there, quite a pleasant discovery was made, there is no more than 1 1/2" of pitch to the patio! I am also beginning to recall tid-bits of a conversation I had with the contractor about our plans to place a tub on the concrete. He mentioned some pitch being necessary but that the tubs water-line needed to be considered. Whew! I think we'll be okay.