The sample tests are wrong in c#, and if those are used in the "real" tests then it is impossible to complete in c#.
This test "worked for me":
[Test]
public void Test1()
{
Assert.AreEqual(new[] { 24, 48 }, Kata.Get_size(4, 2, 6));
Assert.AreEqual(new[] { 1, 1 }, Kata.Get_size(1, 1, 1));
Assert.AreEqual(new[] { 1, 2 }, Kata.Get_size(1, 2, 1));
Assert.AreEqual(new[] { 2, 4 }, Kata.Get_size(1, 2, 2));
Assert.AreEqual(new[] { 100, 1000 }, Kata.Get_size(10, 10, 10));
}