在 ASP.NET Core 3.0 中，可以使用以下代码来实现在发出请求后调用 Program.Main 和 Startup：

Program.cs 文件：

using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Hosting;

namespace AspCore30Example
{
    public class Program
    {
        public static void Main(string[] args)
        {
            CreateHostBuilder(args).Build().Run();
        }

        public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureWebHostDefaults(webBuilder =>
                {
                    webBuilder.UseStartup();
                });
    }
}

Startup.cs 文件：

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using System;

namespace AspCore30Example
{
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            // 添加所需的服务
        }

        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            // 处理请求
            app.Use(async (context, next) =>
            {
                // 在发出请求后调用 Program.Main
                Console.WriteLine("Program.Main is called after request is made.");

                await next.Invoke();
            });

            app.UseRouting();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapGet("/", async context =>
                {
                    // 在发出请求后调用 Startup.Configure
                    Console.WriteLine("Startup.Configure is called after request is made.");

                    await context.Response.WriteAsync("Hello World!");
                });
            });
        }
    }
}

在上述示例中，当发出请求时，会先调用 Program.Main 方法，然后调用 Startup.Configure 方法来处理请求。在控制台中，会打印出相关信息来验证这个顺序。