string abc = "123、ff、ddd、eee、fff、kk、ddd、gggggg、kkk、xxxx";    
string[] x = abc.Split('、');
String xx = "";
for (int i = 1; i < x.Length + 1; i++)
{      
  xx += x[i - 1] + "、";
  if (i % 3 == 0)
  {
    xx = xx.TrimEnd('、');
    xx += "\r\n";
  }
}

------解决方案--------------------

C# code
            string abc = "123、ff、ddd、eee、fff、kk、ddd、gggggg、kkk、xxxx";
            string[] result = Regex.Split(abc, @"(?<=\G(?:[^、]+、){3})");
            foreach (string s in result)
                Console.WriteLine(s);