I found a way to Mod GTA III for Android. Ok, I didn't find a new way. It's the same way as the computer with some little indirrection. Befor I start telling you how to Mod GTA III on Android, I want to warn you.
There are several problems ( other file names, missing dff or txd...).
1. The Mobile gta3.img, which is called gta3_unc.img has got 3859. 11 fewer than the Orginal gta3.img. The vehicles are used from the oldveh_unc.img.
2. The filenames of the IMG files can be various. On my SGSII the are called gta3_unc.img and oldveh_unc.img. But i already saw filenames like: gta3_ptr.img, gta3_atr.img (the same for oldveh_xxx.img)
3. Many people got problems with GTA III (crashes after Loading, or other problems).
4. If you Mod your GTA you may get more problems.
Example: I tested a new Pumpgun on PC. After installation I tried. Works fine.
The same on Mobile. If i press the button for shotting, the game crashes.
5. Dont install 1234567890 at one time or you wont know which mod causes problems and crashes.
6. There are TXD files that can't be opened with TXD viewer applications, because of unknown problems.
If you want to add new Cars, you need to replace the .dff and .txd within oldveh_unc.img.
If you want to Chance Buldings txd's replace the files within gta3_unc.img.
Befor you Mod anything, make a backup of all files or you have to delete files and have to re-download them.
Now let's start with modding.
Download IMG Tool 2.0.
Download the Mod : For an example I use Pegeot 306 Mexican Taxi to replace the Taxi.
Connect your Smartphone to your Computer. Browse to your internal SD-Card e.g.: Computer\GT-I9100\Phone\Android\data\com.rockstar.gta3\files\GTA3\models. Copy oldveh_unc.dir and oldveh_unc.img to the desktop.
Open oldveh_unc.img with IMG Tool 2.0 and replace taxi.dff and taxi.txd.
After this, rebuild the archive with: Commands -> Rebuild Archive.
Exit IMG Tool and copy oldveh_unc.dir and oldveh_unc.img to e.g.: Computer\GT-I9100\Phone\Android\data\com.rockstar.gta3\files\GTA3\models.
Copy handling.cfg from e.g.: Computer\GT-I9100\Phone\Android\data\com.rockstar.gta3\files\GTA3\data to your desktop.
Open handling.cfg with a Texteditor. Replace the lines in the handling.cfg with the one from the readme file.
Copy handling.cfg to e.g.: Computer\GT-I9100\Phone\Android\data\com.rockstar.gta3\files\GTA3\data.
Start GTA III and have fun ( if it works )
Q: When I start GTA III i get a black screen.
A1: Minimize GTA III and then return into the game.
A2: Kill GTA III process with Task-Manager and restart GTA III.
Q: GTA III freezes at the loadingscreen.
A: The Mod you installed won*t work on Android. Install your backups.
Q: I get this error Message when i try to open oldveh_unc.img:
Unable to find oldveh_unc.dir
A: If you try to open a file that is on your handy, your computer copies that file into a temp directory. He only copies the .img, not the .dir file. Repeat Step 2 above.
Screenshots and Videos: