When I first got them, the 3s were weaker than the 2s in the mids and highs. They had a certain silky-smooth tonal quality that I liked but all in all I preferred the 2s. This didn't seem right to me so I did some looking around and found out on Head-Fi that some people had returned their 3s to Audeze for repair, saying that they sounded "veiled". This sounded like mine so I called Audeze and without any trouble at all RMA'd them. They came back repaired with new drivers, and a new graph.
I think the repaired 3s are better than the 2s in most every way, although admittedly not by a lot. I like to say the 2s are amazing, but the 3s are sublime. The real difference is in the feeling they bring to the music. The highs and mids are back and they still have that silky smooth tonal quality. Their bass is stronger yet somehow stays in the background when it isn't meant to be center stage. I'd describe them as more 3-dimensional, subtly bring forward the music's main focus - often vocals. And they have a kind of intimacy that the 2s don't have, making it easier for me to "disappear" into the music.
I still have the 2s but they are available if anyone wants to buy them