using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
		var s = "<td class='additional-attribute'>1</td><td class='additional-attribute'>2</td><td class='additional-attribute'>3</td>";
    	var result = Regex.Matches(s, "(?si)<td class=.additional-attribute.>(.*?)<")
            .Cast<Match>()
            .Take(2)                          // Взять первые два
            .Select(x => x.Groups[1].Value)
            .ToList();
        foreach (var n in result)
        	Console.WriteLine(n);
	}
}

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