//3

Console.WriteLine("Введите цифры: ");
int secret = Convert.ToInt16(Console.ReadLine());
int guess = Convert.ToInt16(Console.ReadLine());

Console.WriteLine($"Быки: {CountBulls(secret, guess)}");
Console.WriteLine($"Коровы: {CountCows(secret, guess)}");

static int CountBulls(int secret, int guess)
{
    return secret.ToString().Zip(guess.ToString(), (s, g) => s == g ? 1 : 0).Sum();
}

static int CountCows(int secret, int guess)
{
    return secret.ToString().Intersect(guess.ToString()).Count() - CountBulls(secret, guess);
}