Hello Lamaringtasa . 
I am here to help you . OK here goes the solution : 
you must open the requested files with notepad for instance right click on the Data\Vehicle.ide and then select  
"open with" then select "notepad" then a notepad file will be opened .  
then replace the stuffs. 
but for opening Models\gta3.img you must download the Image Tool from this site because that file is too  
big for notepad . 
Bye 
Vinewood Boy