在ASP.NET Core 3.0中合并角色的解决方法可以使用ASP.NET Core的身份验证和授权功能。以下是一个包含代码示例的解决方法：

首先，确保已安装了Microsoft.AspNetCore.Authentication和Microsoft.AspNetCore.Authorization NuGet包。

1.创建一个名为"RoleController.cs"的控制器：

using Microsoft.AspNetCore.Authorization;
using Microsoft.AspNetCore.Mvc;

namespace YourNamespace.Controllers
{
    [Authorize(Roles = "Role1,Role2")] // 只允许具有"Role1"或"Role2"的用户访问此控制器
    public class RoleController : Controller
    {
        public IActionResult Index()
        {
            return View();
        }
    }
}

2.在Startup.cs文件中配置身份验证和授权服务：

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

namespace YourNamespace
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

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

            services.AddAuthentication("CookieAuthentication")
                .AddCookie("CookieAuthentication", config =>
                {
                    config.Cookie.Name = "YourAppCookieName";
                    config.LoginPath = "/Account/Login";
                    config.AccessDeniedPath = "/Account/AccessDenied";
                });

            services.AddAuthorization(options =>
            {
                options.AddPolicy("RequireRole1OrRole2",
                    policy => policy.RequireRole("Role1", "Role2"));
            });
        }

        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.UseAuthentication();
            app.UseAuthorization();

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

3.在需要进行角色合并的视图中的HTML代码中使用授权标签：

@using Microsoft.AspNetCore.Authorization

@{
    ViewData["Title"] = "Index";
}

Role Index

@if (User.Identity.IsAuthenticated)
{
    if (User.IsInRole("Role1") || User.IsInRole("Role2"))
    {
        
You have access to this page.
    }
    else
    {
        
You don't have access to this page.
    }
}
else
{
    
You need to log in to access this page.
}

在上述代码中，我们使用[Authorize(Roles = "Role1,Role2")]特性对RoleController进行了授权，只有具有Role1或Role2角色的用户才能访问该控制器。在视图中，我们使用了User.IsInRole("Role1")和User.IsInRole("Role2")来检查当前用户是否具有这两个角色中的任意一个。

请注意，还可以使用自定义角色策略来实现更复杂的角色合并逻辑。以上示例仅演示了一种简单的角色合并解决方法。