from multiple stereo mp2 streams to 6ch ac3 possible?

Hi

I have a transport stream that contains 3 stereo mp2 audio tracks, and
one h.264 video track. I would like to take the 3 stereo mp2 tracks
and turn them into 6 channel ac3, resulting in a transport stream with
1 h.264 video and 1 6ch ac3 audio. Also, I'd have to be able to
re-order the audio tracks to get them into the right order for ac3 as
well. Is there a way to do this with ffmpeg? In realtime?

Thanks,
Andrew