在ASP.NET Core 3中，可以使用IHostApplicationLifetime接口来优雅地关闭应用程序，并在关闭时抛出OperationCanceledException。下面是一个示例代码：

首先，在Startup.cs文件中的ConfigureServices方法中注册IHostApplicationLifetime接口：

public void ConfigureServices(IServiceCollection services)
{
    // 注册IHostApplicationLifetime接口
    services.AddSingleton();

    // 其他服务的注册
    // ...
}

然后，创建一个名为HostApplicationLifetimeWrapper的类，实现IHostApplicationLifetime接口：

using System;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Hosting;

public class HostApplicationLifetimeWrapper : IHostApplicationLifetime
{
    private readonly IHostApplicationLifetime _hostApplicationLifetime;
    private readonly IWebHostEnvironment _hostingEnvironment;

    public HostApplicationLifetimeWrapper(IHostApplicationLifetime hostApplicationLifetime, IWebHostEnvironment hostingEnvironment)
    {
        _hostApplicationLifetime = hostApplicationLifetime;
        _hostingEnvironment = hostingEnvironment;
    }

    public CancellationToken ApplicationStarted => _hostApplicationLifetime.ApplicationStarted;

    public CancellationToken ApplicationStopping => _hostApplicationLifetime.ApplicationStopping;

    public CancellationToken ApplicationStopped => _hostApplicationLifetime.ApplicationStopped;

    public void StopApplication()
    {
        // 在停止应用程序之前执行一些自定义的操作
        // ...

        // 抛出OperationCanceledException以优雅地停止应用程序
        throw new OperationCanceledException();
    }
}

最后，在Program.cs文件中的CreateHostBuilder方法中配置UseShutdownTimeout和UseEnvironment：

public static IHostBuilder CreateHostBuilder(string[] args) =>
    Host.CreateDefaultBuilder(args)
        .ConfigureWebHostDefaults(webBuilder =>
        {
            webBuilder.ConfigureKestrel(serverOptions =>
            {
                // 在这里设置Shutdown Timeout
                serverOptions.Limits.ShutdownTimeout = TimeSpan.FromSeconds(10);
            })
            .UseEnvironment(Environments.Production)
            .UseStartup();
        });

现在，当应用程序关闭时，将抛出OperationCanceledException，以优雅地关闭应用程序。

请注意，根据你的需求，你可能需要根据实际情况进行适当的修改。