Faça como eu fiz: o retorno do Widget

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
        useMaterial3: true,
      ),
      home: Scaffold(
        appBar: AppBar(
          title: Text(
            'Flutter: Primeiros Passos',
            style: TextStyle(color: Colors.white),
          ),
          leading: Icon(Icons.add_task_rounded, color: Colors.white,),
          backgroundColor: Colors.blue,
          elevation: 5,
          shadowColor: Colors.black,
        ),
        body: Container(
          color: Color(0x303aaecc),
          child: Column(
            children: [
              Rainbow(Colors.white, Colors.pink, Colors.lightBlueAccent),
              Rainbow(Colors.redAccent[700]!, Colors.purple, Colors.blue),
              Rainbow(Colors.pinkAccent, Colors.yellow[700]!, Colors.lightBlue),
              Rainbow(Colors.purpleAccent, Colors.white, Colors.green),
            ],
          ),
        ),
      ),
    );
  }
}

class Rainbow extends StatelessWidget {
  final Color color1;
  final Color color2;
  final Color color3;

  const Rainbow(this.color1, this.color2, this.color3, {super.key});

  @override
  Widget build(BuildContext context) {
    const double width = 100;
    const double height = 135;
    return Padding(
      padding: const EdgeInsets.all(10.0),
      child: Row(
        children: [
          Container(
            decoration: BoxDecoration(
              color: color1,
              borderRadius: BorderRadius.circular(20),
              
              border: Border.all(style: BorderStyle.solid, width: 4),
            ),
            width: width,
            height: height,
          ),
          Container(
            decoration: BoxDecoration(
                color: color2,
                borderRadius: BorderRadius.circular(20),
                border: Border.all(style: BorderStyle.solid, width: 4),
            ),
            child: Icon(Icons.people),
            width: width,
            height: height,
          ),
          Container(
            decoration: BoxDecoration(
                color: color3,
                borderRadius: BorderRadius.circular(20),
                border: Border.all(style: BorderStyle.solid, width: 4),
            ),
            width: width,
            height: height,
          )
        ],
      ),
    );
  }
}