string source = "123456789111222333";
            Regex reg = new Regex(@"\d{3}");
            MatchCollection mc = reg.Matches(source);
            foreach (Match m in mc)
            {
                MessageBox.Show(m.Value);//这就是拆分的集合
            }

------解决方案--------------------

C# code
string a1="123456789111222333";
                string[] result = Regex.Split(a1,@"(?<=\G\w{3})").Where(a=>!string.IsNullOrEmpty(a)).ToArray();