As far as I know, the dm-3 is the same as the last version (3205 chip) dm-2, with the addition of the extra output.
I've had both the mn3005 and mn3205 versions of the dm-2 and they both sound great.
I won't say they sound exactly the same, but the differences are not as enormous as some people would make out.
When I decided to sell one of the dm-2s, I kept the mn3005 version, but honestly...
I would have been happy with the mn3205 version just the same.
My DM-2's are the second version with the MN3205 chips, so they probably are pretty similar to the DM-3. I've read that the DM-3 is a bit brighter and noisier, but I believe that description is in comparison to the MN3005 DM-2. I hope to get them all someday! Honestly, I don't think you can lose with any of them though.