It's shared, but it's not the 1 MBps that shared - it's the bigger pipe. The 1 MBps is a cap they artificially put on each end point so one user doesn't hog the whole thing. That's why they can just up it to 3 MBps - apparently utilization is low enough compared to the bandwidth available that they have determined its feasible to do that.
I just got home and ran a test, which I hadn't done for a while, and found that I have been upgraded! I'm getting between 2.7 Mbps and 2.8 Mbps consistently. Cool!
There are hackers who get into "uncapping" their cable modem, but I think the cable companies have become good at detecting them and shutting them down.
I am running in the 2+MBps range on my cable, for the $40 a month. It rocks. No comparison to anything else I have ever used.