using System;

using System.Collections.Generic;

using System.IO;

using System.Linq;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string pattern = @"[.@&]";

        string input = "username@middle&something.else@company.com";

        string inputBeforeLastAt = input.Substring(0, input.LastIndexOf('@'));

​

        // input, pattern, replacement

        string result = Regex.Replace(inputBeforeLastAt, pattern, string.Empty) + input.Substring(input.LastIndexOf('@'));

        Console.WriteLine(result);

    }

}