using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    static void Main(string[] args)

    {

        var parts = 3;

        var rnd = new Random();

        var itemCount = rnd.Next(20);

        //var itemCount = 18; // Value from question post

        var input = Enumerable.Range(0, itemCount);

        var result = input.Split(parts);

        Console.WriteLine($"Splitting {{ {string.Join(", ", input)} }} into {parts} parts:\r\n");

        foreach (var chunk in result)

        {

            Console.WriteLine(string.Join(", ", chunk));

        }

    }

}

​

static class LinqExtensions

{

    public static IEnumerable<IEnumerable<T>> Split<T>(this IEnumerable<T> list, int parts)

    {

        var collection = list is ICollection<T> c

            ? c

            : list.ToList();

        var itemCount = collection.Count;

        if (itemCount < parts)

        {

            yield return collection;

            yield break;

        }

        var itemsInEachChunk = itemCount / parts;

        var chunks = itemCount % parts == 0

            ? parts

            : parts - 1;

        var itemsToChunk = chunks * itemsInEachChunk;

        foreach (var chunk in collection.Take(itemsToChunk).Chunk(itemsInEachChunk))

        {

            yield return chunk;

        }

        if (itemsToChunk < itemCount)

        {

            yield return collection.Skip(itemsToChunk);

        }

    }

}