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The magnetic charging port will only work if connected to a main socket - So if you're thinking of using a USB port from a Pc or in car charging port then it won't work
I have no idea so all I can do is direct you to Xperia care who may be best placed to answer such a question http://www.sonymobile.com/gb/support/contact-us/
All I know from owning one is that it only works when pluged in to a mains socket - I assume it is an amps thing but I'm unable to test this theory - It could also be a monitoring detector in the phone which is able to detect the type of power - Sony themselves have also said it will only work via a mains socket
Look there:
http://talk.sonymobile.com/t5/Xperia-Z1-Z1s/DK31-USB-OTG-at-the-same-time-Is-it-possible/m-p/408517
On Z1 it worked but it doesn't mean it will on Z3. We can't be sure without testing.