You need any sort of tool to view the "gta3" IMG file in the /models folder.

MAKE BACKUPS OF ALL THE MODEL FILES YOU DELETE IN CASE YOU FUCK UP!

Find the model names "CSplay, CSplay2, CSplay3, CSplay4, CSplay5, CSplay6, CSplay8 and CSplay9" 
these are the cutscene models. 
Delete these and add the same files from this folder to the IMG file.

Then find the "player", player2-6,8 and 9 files, these are the in-game textures for the different outfits. 
Again, delete these files and add the ones from this folder.