You will need to download IMG Tool 2.0, then extract the mod files to somewhere, open up IMG Tool, click on File -> Open, then select the gta3.img, next you should have a big list of files. Click on Edit, Find and type in wuzimu.dff or wuzimu.txd, then click on Commands -> Replace, then find the modded .txd or .dff, if you have .dff on IMG Tool, then you must find the modded wuzimu.dff. After you've replaced both of the files, you're done. Oh, and remember; ALWAYS BACK UP YOUR FILES! A lot of people forget this.