There WERE gates seperating third class from the rest of the boat, but those gates were. Literally just waist high fences. They were just there to mark 'hey you're not allowed past here' in universal language, and that was it. It was physically impossible to trap third class passengers in the ship.
What DID happen with a lot of third class passengers was just a case of really unfortunate circumstances - they had the worst steward to passenger ratio, they were the furthest down in the ship, AND they had the widest diversity of languages, giving them the least time to get out, and the worst communication barriers. And even then, y'know what many surviving third class passengers reported?
People did not evacuate third class. They knew the ship was sinking, and they stayed put, because of an overwhelming amount of learned helplessness - for the majority of these people, their lives had NEVER been their own, they were always in the hands of greater powers. So they stayed put, they waited to be ordered to leave, and when those orders never came... they died. Which is fucking awful, but... yeah, not really a design flaw; just the nature of the tragedy.
Anyways, all that aside, I also hate it when people smugly talk about how mOrE LiFeBoAtS would have saved more people, like. Dude, holy shit, the Titanic actually sank EXTREMELY fast, boats that size normally took the better part of a day to sink, she was gone in about two hours. More lifeboats wouldn't have done anything. They didn't even have enough time to launch all the lifeboats they did have - Collapsible A was launched with the sides still down, so all passengers were sitting in ankle deep water all night, meanwhile Collapsible B was never launched at all - it was swept overboard and wound up upside down in the water (with the one surviving radio operator, Harold Bride trapped underneath). The survivors on Collapsible B were mainly men, who managed to climb on top of it, and under the command of Third Officer Lightoller (highest ranked survivor), the men standing on the overturned boat kept it balanced and floating all night, while towing more survivors in the water.
This obviously had high casualty rates, but one of the survivors they towed in the water was the ships head baker Charles Joughin, the final person to leave the Titanic, as he managed to climb up to the top of the stern as it tipped, and calmly rode it down like an elevator while people were panicking and jumping; Joughin didn't even get his hair wet (the Titanic movie also lied about the ship 'sucking people down with it' btw). Joughin also spent the entire sinking drinking heavily, which probably helped him stay calm, and when he was rescued his only health concerns were mild frostbite in his feet, and a raging hangover, god bless.
Anyways as for the lifeboats not being fully loaded, that WAS a mistake, but not the crews fault - that was STANDARD PROCEDURE in the day, lifeboats were NOT meant to be fully loaded while still in the air, you were supposed to put some people in, send them down, the load the rest in the water; loading them in the air would cause their hulls to break from the weight. Now, the lifeboats on the Titanic DID have reinforced hulls, so they actually could be fully loaded in the air, which had held up in testing... but the crew and officers had NOT been informed of that fact. When they were launching full lifeboats near the end, Lightoller's admitted in his inquiry testimony that it was an extremely reckless choice being made out of desperation.
Anyways, if you are also a nerd and want to know how the Titanic sinking actually went down first hand, why not get it right from the primary sources? There was a US Senate Inquiry that began literally the day after the Carpathia arrived in New York, the British held their own inquiry later, and the full transcripts of both can be found here. Extremely interesting stuff - during Gugliemo Marconi's testinmony on the first day, you can literally see the inquiry realizing in real time that hey, why the FUCK isn't it mandatory for ships to have someone at the radio at all times in case of emergency? Really neat stuff.