const string text = @"abc qweabcrty defnmb nmbdef asdabcabcfg";
var rx = new Regex(@"(\w{3})(.*?)\1", RegexOptions.Compiled);
Func<string, IEnumerable<string>> f = null;
f = s =>
{
  var matches = rx.Matches(s).Cast<Match>();
  return matches.Select(m => m.Groups[1].Value).Union(matches.SelectMany(m => f(m.Groups[2].Value)));
};
foreach(var res in f(text).Select(s=>new {Triple = s, Count = Regex.Matches(text, s).Count}).OrderByDescending(t=>t.Count))
  Console.WriteLine("{0} {1}", res.Triple, res.Count);

abc 4
def 2
nmb 2