try this in your code-behind.
In aspx file if you use radio button list for three buttons then use this or if you use radio buttons individual then use
rbtGender.Text
protected void Button1_Click(object sender, EventArgs e)
{
string cs = ConfigurationManager.ConnectionStrings["srinivasdb"].ConnectionString;
SqlConnection cn = new SqlConnection(cs);
SqlCommand cmd = new SqlCommand();
cmd.Connection = cn;
cmd.CommandType = CommandType.Text;
cmd.CommandText = "insert into GenderSample values(@Name,@Email,@Gender)";
cmd.Parameters.Clear();
cmd.Parameters.AddWithValue("@Name", txtName.Text);
cmd.Parameters.AddWithValue("@Email", txtEmail.Text);
cmd.Parameters.AddWithValue("@gender", rbtGender.SelectedValue);
if (cn.State == ConnectionState.Closed)
cn.Open();
cmd.ExecuteNonQuery();
cn.Close();
lblmsg.Text = "Data entered successfully!!!";
}