ascii

int n = int.Parse(Console.ReadLine()!);

int startLetter = 97;

for (int i = 0; i < n; i++)
{
    for (int j = 0; j < n; j++)
    {
        for (int k = 0; k < n; k++)
        {
            char firstLetter = (char) (startLetter + i);
            Console.Write(firstLetter);

            char secondLetter = (char) (startLetter + j);
            Console.Write(secondLetter);

            char thirdLetter = (char) (startLetter + k);
            Console.WriteLine(thirdLetter);
        }
    }
}