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.

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.

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.

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!"

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

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

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.

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

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.

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.

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.

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

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.

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.

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

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 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.