using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        // Organize the strings into a collection, say, an array 

        // (Set, List etc. will do as well)

        string[] lines = {

           "vJrwpWtwJgWrhcsFMMfFFhFp",

           "jqHRNqRjqzjGDLGLrsFMfFZSrLrFZsSL",

           "PmmdzqPrVvPwwTWBwg",

        };

​

        // Query the collection above with a help of Linq

        string commonString = string.Concat(lines

            .Select(line => line.AsEnumerable()) // we deal with IEnumerable<char>, not string

            .Aggregate((s, a) => s.Intersect(a))

            .OrderBy(c => c) // In case you want common characters being ordered

        );

        Console.Write(commonString); 

    }

}