Except that GT2 and maybe GTA1 (never played) had multiplayer in it
I'm going to go out on a limb here and say that the development crew did not use any of the code from previous GTA games in GTA3. I remember the buildings in the previous two installments were 3-D polygonal, but I don't remember if the cars themselves were sprites.
There have been cases (
Alice comes to mind) where a developer takes an engine perfectly capable of multiplayer (or in the case of the
Quake 3 engine, geared towards it) and ships a game with no multiplayer. This means they purposely disabled the multiplayer code or, possibly more drastically, ripped it out entirely.
GTA3 however didn't use the GTA1/2 engine and rip part of it out, it was rewritten from scratch. Ergo, it doesn't matter what the previous installments had, this one doesn't have multiplayer.
Quake 1 & 2 had single player elements - it didn't mean that
Quake 3 had to.
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