Story and contacts
Complete required and available side missions before their points of no return.

Full completion requires planning because changing gang relations can make later activities considerably harder.
One hundred per cent in GTA III is not just about the story. Side missions, 100 hidden packages, 20 rampages, 20 unique stunt jumps, service missions such as Vigilante, Paramedic and Firefighter, and the import/export car lists all count. The game does not show exactly what is missing, so it pays to keep a record from the very first hour.
Gang relations are the biggest trap. Once you turn against a particular group in the story, its territory becomes dangerous, making it much harder to collect packages or complete jumps there. Anyone aiming for a stress-free 100% run should collect everything in each district before falling out with its gang.
Package rewards come in batches of ten: every ten packages you find add a new weapon to your safehouses. Collect from the start and you get equipment for the harder missions free of charge.
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Complete required and available side missions before their points of no return.
Use a numbered list; every ten packages add a reward to the safehouses.
The game must confirm success. A triggered but failed Rampage moves to another location.
Vigilante, Paramedic, Firefighter and Taxi are best completed during the safer early stages.
Vehicle lists and remote-control challenges are among the easiest requirements to overlook.
Compare the individual counters, not only the overall percentage.
If an employer disappeared after progress in another strand, an older save may be necessary.
Do not change main.scm or major gameplay mods during a 100% run.
Answers to the main questions about this topic
One hundred. Every ten you find add a new weapon to your safehouses, so it is worth collecting them as you go.
Yes, both story missions and side missions from employers count towards completion. The story alone is not enough for one hundred per cent — packages, jumps, rampages and service missions will still be missing.
The most common cause is one missing package, jump or rampage. Compare the figures in the Stats menu with the checklist category by category instead of guessing from the map.