要在ASP.NET Core 3中实现依赖注入并使用数据访问/业务层类库，可以按照以下步骤进行操作：

1. 创建一个ASP.NET Core 3项目。
2. 在项目中创建一个名为"DataAccessLibrary"的类库项目，用于实现数据访问逻辑。
3. 在"DataAccessLibrary"项目中创建一个名为"DataAccessService"的类，用于封装数据访问逻辑。

using System;

namespace DataAccessLibrary
{
    public interface IDataAccessService
    {
        void GetData();
    }

    public class DataAccessService : IDataAccessService
    {
        public void GetData()
        {
            Console.WriteLine("Getting data from the database...");
        }
    }
}

4. 在"DataAccessLibrary"项目中创建一个名为"BusinessLogicLibrary"的类库项目，用于实现业务逻辑。
5. 在"BusinessLogicLibrary"项目中创建一个名为"BusinessLogicService"的类，用于封装业务逻辑。

using System;

namespace BusinessLogicLibrary
{
    public interface IBusinessLogicService
    {
        void DoSomeBusinessLogic();
    }

    public class BusinessLogicService : IBusinessLogicService
    {
        private readonly IDataAccessService _dataAccessService;

        public BusinessLogicService(IDataAccessService dataAccessService)
        {
            _dataAccessService = dataAccessService;
        }

        public void DoSomeBusinessLogic()
        {
            Console.WriteLine("Performing some business logic...");
            _dataAccessService.GetData();
        }
    }
}

6. 返回到ASP.NET Core项目，打开Startup.cs文件，并在ConfigureServices方法中添加依赖注入的配置。

using BusinessLogicLibrary;
using DataAccessLibrary;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

namespace YourAspNetCoreProject
{
    public class Startup
    {
        public IConfiguration Configuration { get; }

        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddControllersWithViews();

            // 注册依赖关系
            services.AddTransient();
            services.AddTransient();
        }

        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            else
            {
                app.UseExceptionHandler("/Home/Error");
                app.UseHsts();
            }

            app.UseHttpsRedirection();
            app.UseStaticFiles();

            app.UseRouting();

            app.UseAuthorization();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllerRoute(
                    name: "default",
                    pattern: "{controller=Home}/{action=Index}/{id?}");
            });
        }
    }
}

现在，您可以在控制器或其他地方通过构造函数注入IBusinessLogicService，并使用它执行业务逻辑。

using BusinessLogicLibrary;
using Microsoft.AspNetCore.Mvc;

namespace YourAspNetCoreProject.Controllers
{
    public class HomeController : Controller
    {
        private readonly IBusinessLogicService _businessLogicService;

        public HomeController(IBusinessLogicService businessLogicService)
        {
            _businessLogicService = businessLogicService;
        }

        public IActionResult Index()
        {
            _businessLogicService.DoSomeBusinessLogic();
            return View();
        }
    }
}

这样，您就可以使用依赖注入在ASP.NET Core 3项目中使用数据访问/业务层类库。