I dont have any idea about HTML Agility Pack.
But i can do same thing using Regular Expressions. See my below example, which gets div tags which are having class="header". Same can be done for other classes as well :
string htmlText = "<div class=\"header\">This is Header one</div>"
+ "<div class=\"header\">This is Header two</div>"
+ "<div class=\"header\">This is Header three</div>"
+ "<div class=\"main\">.....</div>"
+ "<div class=\"right\">...</div>"
+ "<div class=\"left\">....</div>"
+ "<div class=\"footer\">...</div>";
var regex = new Regex(@"<div class=""header"">(.*?)</div>");
MatchCollection mc = regex.Matches(htmlText);
List<string> headers = new List<string>();
foreach (Match collections in mc)
{
headers.Add(collections.Groups[0].Value);
}