Needed:-
Txd Workshop
IMG Tool
Paint or any other software just like paint
thats really all you need

1) first open IMG Tool the go to file open and go to C:- Program files - rockstar games - models - then GTA 3
2)Press F2 and extract to your desktop any of your vehicle .txd's you want.
3)Now open .TXD Workshop and open Your txd you just extracted
4)With paint (or other software) find the size of the picture on txd workshop and set it onto paint.
5)Put a high-def or low-def picture /Writing or anything you want onto paint.
make sure it is th right size for the txd.
6)import your picture you just saved onto your desktop onto txd workshop and then if its the body you do the same with all the other metal etc..
7)now after you have done all that you need to save your txd and open IMG Tool.
8)Now Open GTA 3 Again and now find the txd you just modded on IMG Tool.
9)Right Click it and now press replace.
10)find your text you made and then replace the normal .txd
11)You dont have to rebuild archive to do so and now just enjoy your vehicle your style

Creator Tornado
Btw this is my method some other people may make it differently but hope it works for you .