There are many ways inwhich more lives could have been saved, such as people not being as arrogant about the ship being unsinkable, having more lifeboats instead of only enough for half of the crew, hitting the iceberg head on actually wouldn't have sunk the ship, so many things so little time.
The third class passengers were NOT locked in the bottom of the ship like the movie portrays. They were free to move about the ship. They were actually the first to know that something was wrong with the ship since they were at the lowest point of the ship, the scrapping of the ship was extremely loud to them and they ran up top to see what had happened and found large pieces of ice on deck that they started throwing at each other.