1, this test should pass (per the "return empty array when n == 0" rule):
Assert.AreEqual(new double[] { }, Tribonacci(new double[] { 1, 5, 7 }, 0));

2, tests like this should be removed:
Assert.AreEqual(new double[] { 300 }, Tribonacci(new double[] { 300, 5, 7 }, 0));
Assert.AreEqual(new double[] { 0 }, Tribonacci(new double[] { 8, 5, 7 }, 0));

In C#, Attempt returns this EXTREMELY BROKEN response:

Testing for signature: 1, 5, 7 and n: 0

Expected and actual are both <System.Double[1]>
Values differ at index [0]
Expected: 0.0d
But was: 300.0d

Instructions: if n==0, then return an empty array.

So this should pass:
Assert.AreEqual(new double[] { }, Tribonacci(new double[] { 1, 5, 7 }, 0));

Also, if I return a double[] { 0 }, I get the error:

Expected and actual are both <System.Double[1]>
Values differ at index [0]
Expected: 1.0d
But was: 0.0d

If I leave the starting value alone, then I get the first error above.