using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		string[] strings = { "File name without 3 digit end.jpg",
                "File name with 3 digit 123.gif",
                "File name with 3, digit 123.gif",
                "Single 123.jpg",
                "Single.png",
                "File name with ,3 digit 123.gif",
                "Single  123.jpg",
                "Single 1.jpg",
                "Single 123b.gif",
                "More words 123b.png"};

            string pattern = @"^(?!.*[ ]{2})(?!.* ,).*\b(?:\p{L}+|\d{3})\.\w{3}$";
            foreach (String s in strings) {
                if (Regex.IsMatch(s, pattern)) {
                    Console.WriteLine("Match: {0}", s);
                } else {
                    Console.WriteLine("No match: {0}", s);
                }
            }
	}
}