public static string ReplaceWithStars(string s)
{
	if (ReferenceEquals(s, null)) throw new ArgumentNullException("s");
	return s.Length > 3
		? s.Substring(0, 3) + new string('*', s.Length - 3)
		: s;
}

[TestFixture]
public class UnitTests
{
	[Test]
	public void TestReplaceWithStars()
	{
		Assert.AreEqual("abc*****", ReplaceWithStars("abcdefgh"));
		Assert.AreEqual("ABC*****", ReplaceWithStars("ABCDEFgh"));
		Assert.AreEqual("a", ReplaceWithStars("a"));
		Assert.AreEqual("abc", ReplaceWithStars("abc"));
		Assert.AreEqual("abc*", ReplaceWithStars("abcd"));
		Assert.AreEqual("***", ReplaceWithStars("***"));
		Assert.AreEqual("", ReplaceWithStars(""));
		Assert.Throws<ArgumentNullException>(() => ReplaceWithStars(null));
	}
}