Can some one help me implementing ExtentV3HtmlReporter in C#

Since the ExtentV3HtmlReporter (version-3) its pretty old and no longer is being contributed. You could aim for ExtentHtmlReporter (version-4) instead. The version-5 is still in beta.

My csproj with the dependencies:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>

    <IsPackable>false</IsPackable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="NUnit" Version="3.13.1" />
    <PackageReference Include="NUnit3TestAdapter" Version="3.16.1" />
    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.5.0"/>
    <PackageReference Include="Selenium.WebDriver" Version="3.141.0" />
    <PackageReference Include="Selenium.WebDriver.ChromeDriver" Version="90.0.4430.2400" />
    <PackageReference Include="ExtentReports" Version="4.1.0" />
  </ItemGroup>

</Project>

The implementation using Nunit:

using System;
using System.IO;
using AventStack.ExtentReports;
using AventStack.ExtentReports.Reporter;
using NUnit.Framework;
using NUnit.Framework.Interfaces;
using OpenQA.Selenium;
using OpenQA.Selenium.Chrome;

namespace Selenium
{
    public class Tests
    {
        public IWebDriver driver;

        //declare the type of reporter
        public ExtentHtmlReporter htmlReporter;
        public ExtentReports extent;

        //to be used for each of the tests
        public ExtentTest test;

        [OneTimeSetUp]
        public void OneTimeSetUp()
        {

            // One time called before all the tests: create the ExtentReports instance and attach the html reporter 
            // It will create the report.html at the root base of the project
            htmlReporter = new ExtentHtmlReporter("../../../report.html"); //change this path to a location that you want.
            extent = new ExtentReports();
            extent.AttachReporter(htmlReporter);

        }

        [SetUp]
        public void Setup()
        {
            driver = new ChromeDriver(".");
        }

        [Test]
        public void PassedTest()
        {
            //used to create a test entry in the report
            test = extent.CreateTest("Passed Test", "Used just to navigate to an url");

            driver.Navigate().GoToUrl("https://stackoverflow.com");

            //log something
            test.Log(Status.Info, $"After the url was accessed the page title was: {driver.Title}");

        }


        [Test]
        public void FailedTest()
        {
            //used to create a test entry in the report
            test = extent.CreateTest("Failed Test", "Used just to navigate to an url");

            driver.Navigate().GoToUrl("https://stackoverflow.com");

            //log something
            test.Log(Status.Info, "Trying to get an element that will fail");

            driver.FindElement(By.Id("invalid_Id"));
        }

        [TearDown]
        public void TearDown()
        {
            if (TestContext.CurrentContext.Result.Outcome.Status == TestStatus.Failed)
            {
                test.Fail($"Test Failed Successfully with the stack error message: {TestContext.CurrentContext.Result.StackTrace} ");
            }

            if (TestContext.CurrentContext.Result.Outcome.Status == TestStatus.Passed)
            {
                test.Pass("Test Passed Successfully");
            }

            driver.Close();

        }

        [OneTimeTearDown]
        public void OneTimeTearDown()
        {
            //flush to create the report.html after the tests have ran
            extent.Flush();
        }

    }
}