What video outputs does the Comcast provided digital adapter have? If it only has the RF output, you do not subscribe to Comcast HD content and won't be getting any HD channels in the first place. If it does have more outputs (say component, DVI or HDMI), then why are you not using these outputs to your HDTV?
Or are you referring to receiving Over-The-Air HD content as well as Comcast content?
If you want to watch 3D TV at home, there are a few must-have pieces. You'll need a TV capable of displaying 3D video, and special 3D glasses for everyone watching. You'll also need a 3D video source, like a 3D Blu-ray player and 3D Blu-ray movie, as well as an HDMI cable to connect it to your system.