Canon:3e

A major revision of the AD&D rules was released in 2000. As the Basic game had been discontinued some years earlier, and the more straightforward title was more marketable, the word "advanced" was dropped and the new edition was named just Dungeons & Dragons, but still officially referred to as 3rd edition (or 3E for short). It is the basis of a broader role-playing system designed around 20-sided dice, called the d20 system. The 3rd edition removes previous editions' restrictions on class and race combinations that were supposed to track the preferences of the race, and on the level advancement of non-human characters. Level advancement for all characters is greatly eased, allowing players to reasonably expect to reach high level in about one year of weekly play. Skills and the new system of feats are introduced into the core rules to encourage players to further customize their characters.

Monte Cook, Jonathan Tweet, and Skip Williams all contributed to the 3rd edition Player's Handbook, Dungeon Master's Guide, and Monster Manual, and then each designer wrote one of the books based on those contributions.

The d20 system uses a more unified mechanic than earlier editions, resolving nearly all actions with a d20 die roll plus appropriate modifiers. The combat system is greatly expanded, adopting into the core system most of the optional movement and combat system of the 2nd edition Players Option: Combat and Tactics book. Third edition combat uses a grid system, encouraging highly tactical gameplay and facilitating the use of miniatures. The wizard class is divided into Wizards and the new sorcerer class, and in later books such as the Complete Arcane further classes such as warmage are added. The thief is renamed rogue, a term that 2nd edition uses to classify both the thief and bard classes. Third edition also presents the concept of prestige classes, which characters can only enter at higher character levels, and only if they meet certain character-design prerequisites or fulfill certain in-game goals. Later products included additional and supplementary rules subsystems such "epic-level" options for characters above twentieth level, as well as a heavily revised treatment of psionics.

The d20 system is presented under the Open Gaming License, which makes it an open source system for which authors can write new games and game supplements without the need to develop a unique rules system and, more importantly, without the need for direct approval from Wizards of the Coast. This makes it easier to market D&D-compatible content under a broadly recognizable commercial license. Many other companies have produced content for the d20 system, such as White Wolf (under the Sword and Sorcery Studios label), Alderac Entertainment Group, Malhavoc Press, and Privateer Press.

Differences from Advanced Dungeons & Dragons 2nd edition

 * The game system converted to the d20 System, which standardized task resolution to a roll of a 20-sided die ("d20"), adding or subtracting relevant modifiers, and then comparing the result to a "Difficulty Class" (DC) in order to determine the outcome.
 * THAC0 is replaced by a bonus to attack rolls. Armor Class (AC) operates as the Difficulty Class for attack rolls, and therefore increases (rather than decreases, as in 2nd edition) as defensive capabilities increase.
 * Ability scores follow a single table and give standardized bonuses. Ability scores are no longer capped at 25.
 * Saving throws are reduced from five categories (based on forms of attack) to three (based on type of defense): fortitude (constitution-based), reflex (dexterity-based), and will (wisdom-based), and also go up instead of down.
 * "Non-weapon proficiencies" are replaced by skills, and become a fundamental part of the game rather than an optional one, with class abilities such as thieving skills being translated directly into skills. All characters are given a pool of points to spend on a wide range of specific skills to further define a character.
 * Special abilities known as feats allow greater customization of characters. Fighters are no longer differentiated simply by weapons, roleplay and equipment selection, but rather by the number of feats they possess relative to other characters.
 * Magic item creation is simplified, requiring a prerequisite feat, spells, and monetary and experience costs, replacing the obscure rules of earlier editions.
 * Barbarians, monks, and half-orcs return to the Player's Handbook as basic character types.
 * Class groups are removed. "Mage" is renamed to "wizard", with "specialist wizards" being simply wizards that specialize in one school of magic, and "thief" is renamed to "rogue." The bard class is no longer considered a type of rogue.
 * "Priests of a specific mythos", also known as specialist priest classes, are eliminated (except druid), though some make their return in the form of prestige classes or through other options such as feats.
 * The sorcerer class is added to the game as an arcane caster that uses magic naturally, instead of through study.
 * Multi-classing and dual-classing as per previous editions is removed. In the new multi-classing system, multi-classing functioned similar to dual-classing had previously, except that a character could gain a level of any character class upon gaining a level instead of only gaining levels in the second class. Multi-classing is made available to all races, although easier for humans and half-elves, and characters with multiple classes of differing levels are penalized.
 * Prestige classes are added, representing special training or membership in an organization outside the generic scope of core classes. Entry into prestige classes requires characters to meet certain prerequisites. Assassins make their return here, as well as blackguards (fallen paladins) and several others.
 * Any combination of race and class is now permitted, with the exception of some prestige classes. (In 2nd edition, characters of some fantasy races/species are not allowed to belong to some character classes.)
 * Priest spell spheres are removed from the game; each spellcasting class now has its own specific spell list (although wizard and sorcerer share a list). Instead, clerics gain domains that allow them to use bonus spells and abilities based on their deity's area of influence, as well as the ability to swap out prepared spells for curative spells.
 * Initiative is changed to a cyclic system where the order of resolving actions is determined once per encounter and then repeated, and actions are resolved on the players turn. In previous editions the order is redetermined each round and many actions do not resolve on the player's turn but at the end of the round.
 * Diagonal movement and range are simplified. Each square of diagonal distance is equivalent to 1.5 squares of orthogonal distance, rounded down.
 * The system for multiple attacks is changed so that, when making multiple attacks in the same round, later attacks are generally less accurate than earlier attacks.