using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
		var strs = new List<string> { "a d e ads", "H e l l o Everyone", "a b c def", "a b c  def", "Hello world", "a ads", "a d ads" };
		foreach (var s in strs) 
		{
		    var res = Regex.Replace(s, @"(?<!\S)\w(?:\s\w){2,}(?!\S)", m => 
        		new string(m.Value
            	.Where(c => !Char.IsWhiteSpace(c))
            	.ToArray()));
        	Console.WriteLine(res);
		}
	}
}

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