https://twitter.com/i4mwh4ti4m/status/1258768719972454400?s=21

BeforeProgram.cs

// https://twitter.com/i4mwh4ti4m/status/1258768719972454400?s=21
// しすすすすしおおおすし
// 11文字目でおすしがでました

using System;
using System.Collections.Generic;

namespace _3charsCharenge
{
    class Program
    {
        static void Main(string[] args)
        {
            var result = "";
            var rand = new Random();
            List<string> l = new List<string>(){"お", "す" ,"し"};
            var expect = string.Join("", l);

            while(true)
            {
                var c = l[rand.Next(0, 3)];
                result += c;

                if (result.Contains(expect))
                {
                    break;
                }
            }

            Console.WriteLine(result);
            Console.WriteLine($"{result.Length}文字目で{expect}がでました");
        }
    }
}

Program.cs

// https://twitter.com/i4mwh4ti4m/status/1258768719972454400?s=21
// しすすすすしおおおすし
// 11文字目でおすしがでました
using System;
using System.Collections.Generic;

namespace _3charsCharenge
{
    class Program
    {
        static void Main(string[] args)
        {
            var result = "";
            var rand = new Random();
            List<string> l = new List<string>(){"お", "す" ,"し"};
            var expect = string.Join("", l);

            do
            {
                result += l[rand.Next(0, 3)];
            } while (!result.Contains(expect));

            Console.WriteLine(result);
            Console.WriteLine($"{result.Length}文字目で{expect}がでました");
        }
    }
}