The best possible solution (and most likely the only solution which would not require you to also use some 3rd party transcoding software) would be to make Shoutcast v2 + sc_trans2 your "main" stream. Create 3 mount points for it and then configure the autoDJs for each mount point as follows:
mount point1: 128kbps AAC+
mount point2: 96kpbs AAC+
mount point3: the same bitrate and audio encoding type you used to have on your Icecast stream
Then have your Icecast server relay Shoutcat's mount point 3
This does come with a big problem at the moment: most likely you wanted to use MP3 encoding for your 3rd mount point, but sc_trans2 requires an MP3 license to allow you to encode in MP3 format and there is no way to currently purchase an MP3 license from Nullsoft