Is it possible to convert AC-3 to MP3 and keep the surround-information?

no. but you could try another supported format:
http://wiki.hydrogenaudio.org/index.php?title=Multichannel

what is the reason to transcode AC3?


later

smaller audio file in a resulting avi-container+video.
Leaving more rpom to encode the video with, and still maintain (some) surround
sound ?

In that case, use foobar2000 to transcode from AC3 to multichannel Ogg Vorbis at a lower bitrate.

However since Ogg Vorbis is VBR, it won't be compliant with AVI, so I suggest you use another container for your audio/video, such as Matroska.

Yes.
Please correct me if I'm wrong, but I believe that "Prologic II" surround information is stored in a stereo stream.
So you can easily downmix a multichannel ac3 stream into a prologic II stream using free programs like HeadAC3he or BeSweet.

After that, you can encode it to mp3 and it will still have (some) surround information.
I'm not sure how Prologic II is stored in the stream, but I believe that if you keep it stereo (not joint stereo) or even “dual channel” will help to preserve the surround information.