The game was originally pitched to Microsoft by Rockstar to be an exclusive for the Xbox console. However, they turned down the offer believing that the studio wasn't capable, the user interface wasn't that good and that the previous entries in the franchise didn't sell well.

The game's files contain the model for a scrapped vehicle called the "Buggy". It appears to have been scrapped early on, as it is missing a proper body texture with gradients like the rest of the cars in the game, and has damaged parts that are duplicates of the normal body pieces. However, based on its design this vehicle most likely went on to become the BF Injection in Grand Theft Auto: Vice City.

The game was supposed to feature the series' first aircraft missions, but they were removed.

According to a Liberty Tree article, not having the ability to swim in GTA III was because of an oil spill in the Liberty City harbor causing the water to be polluted, not Claude's inability.

The mobile version has some maps leftover from the cut multiplayer feature, suggesting they intended to re-add multiplayer to the mobile port. While they cannot load on the mobile version, with some modding, they can be loaded on the PC version to find that they were mostly finished.

There was originally going to be a fully-fledged first person view for the game. Animations and references to it still exist in the game's files. First person view would not be implemented until Grand Theft Auto V.

All of the songs on the radio station "Flashback 95.6" are from the Scarface film soundtrack.

Through the use of either the Dodo plane or the flying cars cheat, it's possible to get outside the map's boundaries and find a floating street, commonly referred to as the "Ghost Town". According to Rockstar, this was never meant to be accessed through normal gameplay, and was used for the bank heist scene at the start of the game.

An early sketch of Liberty City showed that many different areas were changed during development. Some changes include the airport being in Staunton, the Callahan Bridge not existing, and there were many more roads in Shoreside Vale.

There are strings of unused text that say:
"Get outta here! You're perfectly healthy."
"Healthcare costs."
"I'll just fix you up."
"That will be $250."
This possibly suggests that hospitals at one point may've been enterable to get healed.

Inside the TW@ Internet Cafe are computers displaying screenshots of GTA 1, and GTA 2.

Buskers were planned to make an appearance in GTA III but were dropped during development of the game. Remnants of their existence can be seen within the game files.

Salvatore's model and texture files are named "Frankie" instead of Salvatore.

The name shown on concept artwork of Asuka released by Rockstar Games is shown to say Atsuka.

The working name for Asuka during the development of GTA III was Kemuri. This can be seen in the games internal script, where it states, "~g~Maria and Kemuri are all tied up at the moment -Drop by later!"

An early artwork of 8-Ball has the name Mullan, A.K.A. 8-Ball written besides his image, suggesting his name may have been Mullan in development. This name was never used in any GTA game.

According to the game's code, the Dodo in GTA III is actually a "car" that has been modified to fly (in default.ide, the Dodo's type is set to car, not plane). This is why it has many car-like properties, such as its resilience to high-speed crashes and car-like behavior on the ground. As well the Dodo in GTA III is the only plane in the entire Grand Theft Auto series to feature brakes; these can be applied by using the handbrake key.

The area where Punk Noodles is was originally just a small grass park with two statues. The statue model still exists in the game's files.

A pre-release screenshot shows the police helicopter having an entirely white body color.

The game originally had online multiplayer, but it was cut.

On the teaser site for Capital Auto Sales, it shows many cars that had different names than their in-game equivalent. These are presumably beta names for the respective cars.

The game's graphics were originally more "cartoonish".

The Butler is a cut character. The purpose of the character is unknown, however it's logical that he was intended to be Salvatore Leone's butler. The only remains are the butler's fully textured model in both gta3.img and txd.img.

The German version was heavily censored to meet a USK 16 rating. The edits made were the removal of blood, headshots and amputations taken out (although NPCs would still appear limping if shot), unable to hit anyone whilst they're on the ground (attempting to would have you flail around over them), no money dropped from killed NPCs and all rampage missions removed.

Miguel drives a Banshee for a getaway car in the opening cutscene, which is strange as there are four people in the group and a Banshee has only two seats. This could be hinting Catalina's betrayal.

The Kuruma is referenced in an episode of CBS crime drama "NCIS" where a witness tells Ziva David that the getaway car was a "Kuruma", who gets told by her superior Agent Gibbs that "kuruma" means car, and that her description of the car is car. Agent McGee steps in and tells his confused co-workers, "Kuruma is the name of a car in Grand Theft Auto III, it's a Chrysler Sebring Sedan, boss."

King Courtney's spoken orders in the mission Gangcar Round-up incorrectly state that the Diablo gang car is an Infernus. The subtitles, however, call it a Stallion.

Mr. Wong's Launderette's sign incorrectly spells Launderette as "Laundrette".

The musician Tom Novy was set to appear in the game along with his song "Back To The Streets" but both were cut from final release. However, his model can still be found in the files.

Originally the Triads had black outfits and tattoos.

Early screenshots show that buses were originally supposed to appear in-game.

Children and elderly people with walkers were planned for the game, but were cut.

Claude could originally run with the baseball bat.

Some early copies of GTA III still featured the logo of DMA Design, the original name of Rockstar North, during the opening credits.

The beta version of GTAIII's HUD had a life bar instead of percentages.

Ads can be spotted all over the city for "Top Down City", starring Arnold Steelone (a reference to famous action film actors Arnold Schwarzenegger and Sylvester Stallone). The film is a reference to Grand Theft Auto 1 and 2, as these games are in the top-down perspective.

Officially, the protagonist's name in this game is "Claude". Hacking into the game's files, one can find a text string saying "DEFNAME = CLAUDE" which not only made it clear his name was Claude, but also hinted at a removed mechanic allowing the player to rename the main character. As a result, many also believe he is Claude Speed, the protagonist of GTA2.

Sam Houser claimed that the GTA3 protagonist did not have a name as he was merely the player's avatar, but would later be named "Claude" as of San Andreas, as everyone had taken to calling him that.

There were plans of putting a homeless man named Darkel in the game. His missions were to supposedly have you blow up buildings. He was removed after 9/11 attacks, possibly due to the developers thinking Darkel's character was in bad taste. Despite this, Darkel still remains listed in the manual credits and his character remains in the game's data.

Rockstar claimed that Darkel was removed because he did not tonally fit with the rest of the game, and his removal wasn't related to 9/11.