Adding new weapons ain't just some fancy editing in files.If you want to add new weapons, only way is to mod your game.Ok so I'll tell you what to do:
-Download IMG Tool.
-Download the weapon files you want [usually you find it in a .rar archive]
-Extract the weapon files in a folder. [weapons file are weapon.dff and weapon.txd <- instead of 'weapon', it'll be the weapon name, eg: ruger.dff/ruger.txd]
-Open IMG Tool, and open GTA3.img which you find in the models folder.
-Search in the GTA3.img for the weapon's .dff and .txd, and make a backup [recommended]
-After making a backup of the two files, delete them from the .img file
-Then, click on the 'Commands' tab, and select 'Add'
-Search for the downloaded .txd and .dff files [should be in the folder you extracted them from the .rar archive]
-After the files are added, click on 'Commands' tab, click 'Rebuild Archive', and wait for it to rebuild [should take around 60-70 seconds]
-Then start the game and see if it worked [if it didn't work, best is to delete them from the .img file and replace them with the backed up files]
Hope I helped.I kinda forgot what to do with .col files, since deleted the game some months ago, but the only way to add weapons, as far as I know, is by modding and replacing old.No other weapon can be added.