If Germany had sensible immigration laws (including birthright citizenship), there wouldn't be any "third-generation Turkish immigrants" voting in Turkish elections. They would have assimilated into German society and not be Turkish citizens.
Turkey gives citizenship to anyone born to a Turkish parent (or parents) abroad regardless of the other nationalities the person might acquire at birth, so it doesn't matter if they have German citizenship or not as long as their parents haven't actively renounced their Turkish citizenship.