So, you'll need IMG Tool 2.0 and Notepad. Open IMG Tool and press Ctrl+L, then locate your gta3.img and click Open. You should have a big list of .dff's and .txd's, no worries. Press F2 and a small window should show up. Type in sanchez.dff and click Find Next. You should then have something look a little like this:
Press Ctrl+R and locate the sanchez.dff that JVT has provided. It should then load for a bit, and it should work. Now for the tricky part. Open your handling.cfg and default.ide which can both be found in the data folder. Open the default.ide and it should open it with Notepad, if it doesn't, then right-click on it, and it should say Open With, choose Notepad. Scroll down and look for a line similar to this:
198, sanchez, sanchez, bike, DIRTBIKE, SANCHEZ, biked, ignore, 7, 7, 0, 16, 0.65
Copy the code that I just gave you (which is the same as JVT's), highlight the Sanchez line and press Ctrl+V. So, that's that part over. Now follow the same process again except this time open the handling.cfg and search for a line similar to this one:
DIRTBIKE 1100.0 1.2 1.8 1.5 0.0 0.05 -0.09 103 1.6 0.9 0.48 5 190.0 50.0 R P 19.0 0.50 0 35.0 0.85 0.15 0.3 0.15 10000 0.15 -0.16 0.5 0.0 10003 1 1
Copy the one I just gave you and highlight the original handling line. With that highlighted, press Ctrl+V to replace it with the new one. Click close and save it. Remember to always back up your files before you attempt to mod them! So, after all that, start the game to see if it works. If it doesn't, post back here and we'll try to assist you. Welcome to America The GTA Place.