using System;

using System.Collections.Generic;

using System.Linq;

using System.Text.RegularExpressions;

​

public class Program

{

    public class Item

    {

        public string Col1;

        public string Col2;

        public string Col3;

        public Item(string col1, string col2, string col3)

        {

            Col1 = col1;

            Col2 = col2;

            Col3 = col3;

        }

    }

​

    public static void Main()

    {

        var Object = new List<Item>{

            new Item("0000", "Item1", "P11"), 

            new Item("0000", "Item0", "1"), 

            new Item("0000", "Item0", "10"), 

            new Item("0000", "Item1", "P3"), 

            new Item("0000", "Item0", "12"), 

            new Item("0000", "Item0", "3"), 

            new Item("0000", "Item0", "2"), 

            new Item("0000", "Item1", "P12"), 

            new Item("0000", "Item1", "P1"), 

            new Item("0000", "Item1", "P10"), 

            new Item("0000", "Item1", "P2"), 

            new Item("0000", "Item0", "11")

        };

        var orderedCustomers = Object

            .OrderBy(c => c.Col1)

            .ThenBy(c => c.Col2)

            .ThenBy(d => TxtPart(d.Col3))

            .ThenBy(d => NumPart(d.Col3));

        foreach (var x in orderedCustomers)

            Console.WriteLine(x.Col1 + ", " + x.Col2 + ", " + x.Col3);

    }

​

    // Helper methods

    public static string TxtPart(string s) => Regex.Match(s, @"^[^0-9]*").Value;

    public static int NumPart(string s) => int.TryParse(Regex.Match(s, @"[0-9]*$").Value, out int v) ? v : int.MaxValue;

​

}