This only concerns Vice City, I have little expertise in GTA3:
Pros
MUCH less crashes, though unfortunatly the ones that occur do at the most inconvenient times (usually interacting with another player, example a car jack power struggle usually does it).
New features
Not 24/7 stubby wars
Cons
Worse gameplay with more desyncing happening and less people playing
Some weapons are now squirt guns, and I dont mean cause the damage was altered (m60 being an example)
More desync (sometimes stunt mode gets VERY desynced, death match being disappearing players)
Chrome Shotgun somewhat takes the place of a stubby, players can usually be killed with it like a stubby if not at full health.
Helicopters reduced COMEON
Yes I do believe .3 was better