matrix and functions

using System;

public class Program
{
    public static void Main()
    {
        int[] arr = {1,2,3}; //same: int[] arr = new int[] {1,2,3};
        int[,] mat = new int[3, 3];
        int[][] jagged_arr = new int[3][];
        int[][] jagged_arr2 = {
            new int[] {1, 2, 3, 4},
            new int[] {11, 34, 67},
            new int[] {89, 23},
            new int[] {0, 45, 78, 53, 99}
        };

        mat = fill_mat(mat);
        Print2DArray(mat);

        jagged_arr = fill_jag(jagged_arr);
        PrintJaggedArray(jagged_arr);
        PrintJaggedArray(jagged_arr2);

        //3D array
        Console.WriteLine("\n3D array");
        int[, ,] threeDimensional = new int[3, 5, 4];

        //fill all the layers
        for(int x = 0; x < threeDimensional.GetLength(0); x++)
            for (int y = 0; y < threeDimensional.GetLength(1); y++)
                for(int layer = 0; layer < threeDimensional.GetLength(2); layer++)
                    threeDimensional[x, y, layer] = layer+1;


        //print all the layers
        for (int i = 0; i < threeDimensional.GetLength(2); i++)
        {
            for (int y = 0; y < threeDimensional.GetLength(1); y++)
            {
                for (int x = 0; x < threeDimensional.GetLength(0); x++)
                    Console.Write(threeDimensional[x, y, i]);
                Console.WriteLine();
            }
            Console.WriteLine();
        }
    } // end

    static int[,] fill_mat(int[,] mat){

        mat = new int[,] {
            {1,2,3},
            {3,4,5},
            {5,6,7}
        };

        return mat;
    }

    static void Print2DArray(int[,] arr)
    {
        Console.WriteLine("\nMatrix:");

        // Display the array elements.
        for (int i = 0; i < arr.GetLength(0); i++)
            for (int j = 0; j < arr.GetLength(1); j++)
                Console.WriteLine("Element({0},{1})={2}", i, j, arr[i, j]);
    }

    static int[][] fill_jag(int[][] arr)
    {
        arr[0] = new int[] {1, 2, 3, 4};
        arr[1] = new int[] {1, 2, 3, 4};
        arr[2] = new int[] {1, 2, 3, 4};

        return arr;
    }

    static void PrintJaggedArray(int[][] arr)
    {
        Console.WriteLine("\nJagged:");

        for (int n = 0; n < arr.Length; n++)
        {
            // Print the row number
            System.Console.Write("Row({0}): ", n);

            // Print the elements in the row
            for (int k = 0; k < arr[n].Length; k++)
                System.Console.Write("{0} ", arr[n][k]);

            System.Console.WriteLine();
        }
    }
}