Here's my in depth analysis of the files (special thanks to Marphy for helping me write this message).
These are the "help file entries" to instruct the level designers how to use the entities defined in the "game configuration files" (*.fgd files) used by the level editor. They can't be used to build a level for a game. The files were made public, possibly unintentionally, by Valve on October 9, 2015, a day before the 8th anniversary of The Orange Boxs release in 2007.
hl3.txt
Most of the code data is from Half-Life 2, and it was ported over to Source 2 with changes and additions. Here are some of the features:
> Base AI functions from Half-Life 2 (companions, squad following, attack and cover, scripted sequences).
> Possible quest system (info_quest_dialog, point_quest_goal).
> Virtual reality settings in the map properties (enable VR Chaperone, VR Movement).
> Character entities are: npc_quest_citizen, npc_hunter_invincible, npc_turret_ceiling_pulse (Combine Pulse Ceiling Turret).
> Other entities are: props_zipline, item_generic.
> "HL3 only" prefix is used two times in the document.
Possible Left 4 Dead style AI Director for procedural generated enemy placements. It is similar to the version used in Left 4 Dead 2. The level designers have more control over the spawns.
> Feature: Level designers can create a spawn template and reuse it.
> Feature: Spawn radius (inside or outside the player's radius), spawn limit.
> Feature: Visibility requirement (to prevent the enemy spawning in front of the player).
Name (in the code): Attribute.item_generic.associated_player_counter
Name (in the help file): Associated PlayerCounter Name (HL3 only)
Explanation: This is an option for item_generic. The playercounter is a possible hint of a game mode with multiple players.
Name (in the code): Attribute.item_generic.pickup_prevents_respawning
Name (in the help file): Pickup prevents respawning (HL3 only)
Explanation: This is an option for item_generic. Another possible hint of a game game mode with multiple players. This option is used to disable to prevent the item from respawning.
Name (in the code): Attribute.npc_quest_citizen.is_quest_member
Name (in the help file): Quest Member? (Neutral to Combine and doesn't take damage)
Explanation: This is an option for "npc_quest_citizen" entity. When enabled, the character becomes friendly/neutral to the Combine units and doesn't take any damage from their weapons.
Name (in the code): Attribute.npc_quest_citizen.beckon_approach_enabled
Name (in the help file): Beckon and approach the player
Name (in the code): Attribute.npc_quest_citizen.beckon_radius
Name (in the help file): Beckon the player if they come within this radius
Name (in the code): Attribute.npc_quest_citizen.approach_radius
Name (in the help file): Approach the player if they come within this radius
Name (in the code): Attribute.npc_quest_citizen.combat_enabled
Name (in the help file): Allow this citizen to participate in combat
rpg.txt
This appears to be a base for a single player (keywords: client, globalname) role-playing game. It is not related to hl3.txt since it has its own definitions for map properties. There are generic character and item spawners, but no game data is present. It might be a base for a cancelled/unannounced Valve game or an experiment. There is also a navigation system, similar to the one used by the bots in Team Fortress 2 and Left 4 Dead games.
Entity: rpg.unit
Description: A generic character with basic AI functions. They can work in squads like Half-Life characters. They have a feature called lurking behaviour.
Entity: rpg_item_spawner
Description: Generic item spawner.
Entity: rpg_prop_itemdrop
Description: This appears to be a breakable prop the characters can interact with. When destroyed, it leaves rpg_physical_items behind.
Entity: rpg.spawner
Description: It is used for spawning entities.
Entity: rpg.trigger
Description: It is used for triggering entities.
Entity: rpg_gamerules
Description: It is used to edit the rules of the game.
lights.txt
This is a base file, a special type of file that contain the base entities (i.e. func_wall) used by the engine, and it must be used with a game configuration file to create a map for a game.
Entity: timeofday
Description: To set the time of the map
Note: This is a base feature of Source 2. It doesnt mean it is used for Half-Life 3.
Entity: realisticdaynightcycle
Description: For creating realistic day and night cycles. You can set the day/month/year, the length of the day. It is similar to the technology used in the Grand Theft Auto games.
Note: This is a base feature of Source 2. It doesnt mean it is used for Half-Life 3.
ai_basenpc.txt
Another base file used for AI functions ported from Half-Life 2.
Entities: ai_addon, ai_addon_builder
These entities are possible related to the Half-Life 2 code references (see below) found in the leaked Dota 2 client from 2011. ai_addon_builder (similar to npc_maker, but more complex) appears to be an entity that can spawn characters with addons (items). ai_addon is a definition file for the items.
dota/staging/src/game/server/hl2/addon_basecombatweapon.cpp
dota/staging/src/game/server/hl2/addon_baseshooter.cpp
dota/staging/src/game/server/hl2/addon_bomb.cpp
dota/staging/src/game/server/hl2/addon_brain.cpp
dota/staging/src/game/server/hl2/addon_dartgun.cpp
dota/staging/src/game/server/hl2/addon_minigun.cpp
dota/staging/src/game/server/hl2/addon_mortar.cpp
dota/staging/src/game/server/hl2/addon_propeller.cpp
dota/staging/src/game/server/hl2/addon_rpg.cpp
dota/staging/src/game/server/hl2/addon_saw.cpp
dota/staging/src/game/server/hl2/addon_shield.cpp
dota/staging/src/game/server/hl2/addon_thrownprojectile.cpp
dota/staging/src/game/server/hl2/ai_addon_builder.cpp
dota/staging/src/game/server/hl2/ai_addon_factory.cpp
