Just get MySQL to do the deletion at the same time as identifying the records to delete
string constr = @"Data Source=192.168.*.**; Database=wsditsupport; User ID=****; Password=*****";
using (MySqlConnection con = new MySqlConnection(constr))
{
using (MySqlCommand cmd = new MySqlCommand("delete from datalogin where datediff(now(), datedown) > 3;",con)
{
cmd.ExecuteNonQuery();
}
con.Close();
}