ITestAction placed on assembly level does not run before and after each test fixture. Place the nunit 3 assemblies in the directory of this gist, and run 'run.bat'.

Run.bat

@echo off
csc Test.cs /target:library /debug+ /debug:full /reference:nunit.framework.dll
nunit3-console.exe Test.dll

Test.cs

[assembly: TestActions.MyTestAction]

namespace TestActions {
    using System;
    using System.IO;
    using System.Threading;

    using NUnit.Framework;
    using NUnit.Framework.Interfaces;

    [AttributeUsage(AttributeTargets.Assembly)]
    public class MyTestActionAttribute : Attribute, ITestAction {
        public void BeforeTest(ITest test) {
            TestLog.Instance.Log("Before: " + test.FullName);
        }

        public void AfterTest(ITest test) {
            TestLog.Instance.Log("After: " + test.FullName);
        }

        public ActionTargets Targets => ActionTargets.Suite | ActionTargets.Test;
    }


    public sealed class TestLog : IDisposable {
        private static readonly Lazy<TestLog> LazyInstance = new Lazy<TestLog>(() => new TestLog(), LazyThreadSafetyMode.ExecutionAndPublication);

        public static TestLog Instance => LazyInstance.Value;

        private readonly StreamWriter _log;

        private TestLog() {
            this._log = new StreamWriter(Path.Combine(Path.GetDirectoryName(typeof(TestLog).Assembly.Location), "TestLog.txt"), false);

            AppDomain.CurrentDomain.DomainUnload += (_, __) => this.Dispose();
        }


        public void Log(string msg) {
            this._log.WriteLine(msg);
            this._log.Flush();
        }

        public void Dispose() {
            this._log.Flush();
            this._log.Close();
        }
    }

    [TestFixture]
    public sealed class MyTestFixture {
        [Test]
        public void Test1() {
            TestLog.Instance.Log("MyTestFixture.Test1");
        }

        [Test]
        public void Test2() {
            TestLog.Instance.Log("MyTestFixture.Test2");
        }
    }
}