Once the arcade board's NVRAM was first initialised (or "powered on" for the first time), an internal timer would begin. Then, according to the Tekken 3 wiki on StrategyWiki, This means that if you started a brand-new arcade cabinet today, you would only have a small selection of fighters available. The rest would unlock automatically, one per week, for nearly three months! The unlock order is typically as follows:
Check if your nvram folder is read-only. Also, ensure you are exiting MAME via the ESC key or the "Exit" menu command. Forcing the emulator closed (killing the process via Task Manager) prevents MAME from writing the NVRAM data back to the disk.