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I'm sorry to hear that, contact your local Xperia care regarding this issue
http://support.sonymobile.com/global-en/contactUs/
Also keep in mind that the warranty only cover the country it was originally purchased from.
I don't know the reason but this is my opinion, so don't quote me on that.
Perhaps they don't sell the Z3 only the Z3 Dual, perhaps legally they can't repair your phone unless they sell that model in that specific country.
If you don't care about warranty, you could replace the screen yourself, it's a lot cheaper, but it will void your warranty, and of course there's a risk that you might damage the phone.