求个轮换正则

求个替换正则
url如下
News22_33_55.shtml
News1_2_3.shtml
News4_5_6.shtml

怎么把第三个参数即“_55”,“_3”,“_6”替换成空字符串

[解决办法]
str="News22_33_55.shtml";
str=str.Replace("_55", "").Replace("_3", "")..Replace("_6", "");
[解决办法]
Regex.Replace("News22_33_55.shtml",@"_[^_]+(?=\.)","")
[解决办法]