Credits are used to move through the upload queue of the other client, not for downloading. More credit you have with him - faster you advance in his queue. Maybe advancing three times faster is what you want, but from your post I don't think so.
Should it mean start downloading the file instantly? Did you ever think it over? 3% of a 4GB file is 120MB!
Say, 1 release a 4GB file, 20 ppl are downloading it from me. In case they reach the critical last 3% in the same time ("same time" depends on connection, let's say within one day), I'd start forced upload of 120 MB to 20 ppl, and it's just one file! What if I share 100 big rare files?
Personal note: If i'd ever notice my aMule does something like I described above, I'd instantly kill it and not turn it on again for at least a week, and then I'd consider removing that file from my share.
