main.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Welcome to Flutter',
      home: Scaffold(
        appBar: AppBar(
          title: Text('Timer'),
        ),
        body: Center(
          child: Column(
            mainAxisSize: MainAxisSize.min,
            children: <Widget>[
              Row(
                mainAxisAlignment: MainAxisAlignment.center,
                children: <Widget>[
                  CustomTextContainer(label: 'HRS'),
                  CustomTextContainer(label: 'MIN'),
                  CustomTextContainer(label: 'SEC'),
                ],
              ),
              Container(
                margin: EdgeInsets.only(top: 20),
                child: RaisedButton(
                  child: Text('START'),
                  onPressed: () {

                  },
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}

class CustomTextContainer extends StatelessWidget {

  CustomTextContainer({this.label});

  final String label;

  @override
  Widget build(BuildContext context) {
    return Container(
      margin: EdgeInsets.symmetric(horizontal: 5),
      padding: EdgeInsets.all(20),
      decoration: new BoxDecoration(
        borderRadius: new BorderRadius.circular(10),
        color: Colors.black87,
      ),
      child: Column(
        mainAxisSize: MainAxisSize.min,
        children: <Widget>[
          Text(
            '00',
            style: TextStyle(
              color: Colors.white,
              fontSize: 54,
              fontWeight: FontWeight.bold,
            ),
          ),
          Text(
            '$label',
            style: TextStyle(
              color: Colors.white70,
            ),
          )
        ],
      ),
    );
  }
}