import 'package:flutter/material.dart';

                    void main() {
                      runApp(
                        MaterialApp(
                          home: Scaffold(
                            appBar: AppBar(
                              centerTitle: true,
                              title: Text('I\'m an App'),
                              backgroundColor: Colors.red[600],
                            ),
                            body: App(),
                          ),
                        ),
                      );
                    }

                    class App extends StatelessWidget {
                      @override
                      Widget build(BuildContext context) {
                        return Container();
                      }
                    }

Change StatelessWidget

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Column(
        mainAxisAlignment: MainAxisAlignment.spaceAround,
        children: <Widget>[
          // Row 1
          Row(
            children: <Widget>[
              Container(
                color: Colors.blue, height: 40, width: 40, child: Text('1')),
              Container(
                color: Colors.blue, height: 40, width: 40, child: Text('2')),
              Container(
                color: Colors.blue, height: 40, width: 40, child: Text('3')),
            ],
          ),
          // Row 2
          Row(
            children: <Widget>[
              Container(
                color: Colors.blue, height: 40, width: 40, child: Text('1')),
              //Will expand to fill all remaining space
              Expanded(
                  child: Container(
                    color: Colors.green,
                    height: 40,
                    width: 40,
                    child: Text('2'))),
              Container(
                color: Colors.blue, height: 40, width: 40, child: Text('3')),
            ],
          ),
          //Row 3
          Container(
              height: 100,
              child: Row(
                //Stretches to vertically fill its parent container
                crossAxisAlignment: CrossAxisAlignment.stretch,
                children: <Widget>[
                  Container(
                    color: Colors.blue,
                    height: 40,
                    width: 40,
                    child: Text('1')),
                  Expanded(
                      child: Container(
                        color: Colors.green,
                        height: 40,
                        width: 40,
                        child: Text('2'))),
                  Container(
                    color: Colors.blue,
                    height: 40,
                    width: 40,
                    child: Text('3')),
                ],
              )),
          // Row 4
          Row(
            //Creates even space between each item and their parent container
            mainAxisAlignment: MainAxisAlignment.spaceAround,
            children: <Widget>[
              Container(
                color: Colors.blue, height: 40, width: 40, child: Text('1')),
              Container(
                color: Colors.blue, height: 40, width: 40, child: Text('1')),
              Container(
                color: Colors.blue, height: 40, width: 40, child: Text('3')),
            ],
          )
        ]);
  }
}