string ary = "111";
Console.Write(Regex.IsMatch(ary, "[0-9]{3}")); //True

------解决方案--------------------
C# code
Regex x=new Regex("[0-9]{3}"); 
 			string s="111"; 
 			if(x.IsMatch(s)) 
 			{ 
 				MessageBox.Show("true"); 
 			}