Boxes that are in the center boxes [ copy and past code into dartPad ]

gistfile1.txt

import 'package:flutter/material.dart';
void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Row(
          children: [
            Container(color: Colors.red, width: 100),
            Expanded(
              child: Container(
                color: Colors.teal,
                child: Column(
                  mainAxisAlignment: MainAxisAlignment.center,
                  children: [
                    Container(color: Colors.yellow, width: 100, height: 100),
                    Container(color: Colors.yellow.withAlpha(125), width: 100, height: 100),
                  ],
                ),
              ),
            ),
            Container(color: Colors.blue, width: 100),
          ],
        ),
      ),
    );
  }
}