Actually,Fatal Fury 3 PC's cheat code is same as Neo.Geo version.

1.To use Ryuju Yamazaki and Jin Chon Shu and Jin Chon Rei

You have to finish game and save the data which u've already met Ryuju Yamazaki and Jins' Bro.Choose "load savedata" when u start game next time,and choose "new game".In the characters choose screen,move the choose box with the order below,and press Low Kick button once on every one:

Terry->Hon Fu->Mai->Geese->Bob->Sokaku Mochizuki->Andy->Franco->Joe
Higashi->Blue Mary

If you input this successfully,the "here come new fighters" will pop up,and now you can use Ryuju and Jins' bro.

2.To use Tiny Fighters in VS mode

In VS mode,Hold both 1P and 2P 's Low Kick and High Punch key when confirm your fighters