Realize that Canon introduces "changes" to their Raw format that makes them incompatible with existing software.
Canon works on their Raw processing software, Digital Photo Professional (DPP), and releases a new version on disk with your camera that actually does a nice job of processing and converting your Raw files.
Other third-party vendors have to wait until they receive a camera, and then reverse-engineer things to be able to properly render the Raw data into a usable image format.
Adobe does this on an ongoing basis -- they update Camera Raw every three months or so. But, when they roll out a new version of CSx, they move support into that version, and stop supporting previous versions. Complain if you wish, but they are "in business"...
But they did release the DNG format, which is how you can convert a Raw file to be compatible with earlier Raw processors. You get the converter which is compatible with your camera and you set it to be compatible with Camera Raw 4.6, and you are off and running.
So, in a nutshell:
You can convert to DNG and use CS3.
You can use DPP to process the Raw and then transfer to Photoshop if/when needed
You can use a third-party software, such as Lightroom or others to handle your Raw processing (Lightroom also handles much more) and use Photoshop as needed
Or, of course, you can update to Photoshop CS5 -- there are some nice features in there, but at a price!