How do I set the initial page of PageView in Flutter?

PageController constructor has named parameter initialPage. You can use it, just create the controller somewhere outside of build function:

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  PageController controller;

  @override
  void initState() {
    super.initState();
    controller = PageController(initialPage: 3);
  }

  @override
  void dispose() {
    controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: SafeArea(
          child: PageView(
            controller: controller,
            children: [
              for (var i = 0; i < 4; i++)
                Text('test $i')
            ],
            scrollDirection: Axis.vertical,
          )
        )
      ),
    );
  }
}