01.05.2013, 15:15
as far as i know, those .dff and .col files belong to each other, so place all of the 2 or 3 files per object into the .img file. dont forget to rebuild the archive after you modded it, it should go fast, since you are not modding the gta3.img 
after you built the .img file freshly (it grows up in size of course), also add the new objects to the .ide file, so the engine can associate an ID to them. you already figured out to do that, so maybe most of the stuff i mentioned is irrelevant.
compare the GX img file with yours, and the .ide, maybe you find some differences (an id missing in your ide? .col and .dff added and rebuilt?)
btw, only real nerds spend may 1st at the PC, have patience, and fucking WAIT for an answer. dont expect it today. all i can give, are some regular hints - i didnt touch the img for years, should do that again, there are some "hell race" objects :P
after you built the .img file freshly (it grows up in size of course), also add the new objects to the .ide file, so the engine can associate an ID to them. you already figured out to do that, so maybe most of the stuff i mentioned is irrelevant.
compare the GX img file with yours, and the .ide, maybe you find some differences (an id missing in your ide? .col and .dff added and rebuilt?)
btw, only real nerds spend may 1st at the PC, have patience, and fucking WAIT for an answer. dont expect it today. all i can give, are some regular hints - i didnt touch the img for years, should do that again, there are some "hell race" objects :P