AFAIK, HEX-editors should work, but that is a pretty cumbersome thing to employ, and practically impossible unless you know the precise hex adresses... What should do, is unpacking ISO, changing some files, and boiling it back again... AS LONG as these files are in correct format, BUT - most are packed, and I know no tools to unpack them. =\
Honmaru - Tekken 3 Online Center
These guys created a Online hack of Tekken 3... Even though it uses a PC online client for emulator, but the ISO is still hacked...
YouTube - Tekken 3 Online: Tan (EST) vs. Bury - part1
YouTube - TEKKEN 3 VOL.4 (JAP)
See - someone redraw certain Tekken 3 textures, EVEN that they are all packed - it is just few TIM images for textures (yeah, you can view and extract them with certain programs, and I've seen that they are cut in pieces), but I am afraid that 2D sprites are too cumbersome to cope with.
Guys - can anyone shed some more light on how to make these mods? =\
P.S. I've made a similar thread before, you can check it if you like:
PSX Games Modding/Texture Editing