I think there isn't a way to convert tracks. I tried to modify the t3ed sometime ago, but it seemed to be too much work and I gave up. Are there really nfs3 track that are worth to be converted ?
Do the tracks in NFS 3 use the same TR.FRD and T00.TGA format? If so maybe one can just copy them over to a NFS 4 track, Keeping all files the same except for the TR.FRD and T00.TGA?
I remember that Nappe1 managed to port one of his earliest NFS3 tracks (Granite Mountains) to NFS4. Sadly, I don't have that track anymore (another one on the "I thought I did... but I guess I don't" list), so I can't give you a look.I don't know if he found a way to actually convert it, or that he managed to re-create it in NFS4 format, but according to the Internet Archive: https://web-beta.archive.org/web/20010604173256/http://rp-design.totalnfs.net:80/index.shtml the converting part was done by Dan2K from West Coast Design...Sorry that this all doesn't really help.