If these were larger dealers they may also have detailed/cleaned the engine, though since I'm new to Porsche and the engine is out of view I don't even know if that is a common practice. Or maybe they all have new tires and you are heating up some rubber
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Or like Iflylow is saying all three have had the bad luck of having a spark plug tube seal leaking.
Find a private sale and drive one or take the one you like most to an indy and have them tell you. Could also be your cash burning or your increase in sensory after driving one of these machines
Let us know what you find out.