TripleSum.cs

T[] TripleSum(T[] first, T[] second, T[] third)
{
    T[] output = new T[first.Length];
    for (int i = 0; i < first.Length; i += Vector<T>.Count)
    {
        Vector<T> a = new Vector<T>(first, i);
        Vector<T> b = new Vector<T>(second, i);
        Vector<T> c = new Vector<T>(third, i);

        Vector<T> sum = a + b + c;
        sum.CopyTo(output, i);
    }

    return T[];
}