I own both an an RS chassis'd SD and a TC3. I don't however own a CGM.
The SD is a much nicer to car to work on [than a TC3] - it's far quicker and easier to service and I personally prefer metric screws etc as I am more familiar with them.
The TC3 is a hard car to beat performance wise. Straight out of the box, I'd still say the TC3 FT is the car to beat - though I think things are only just beginning to change.
The CGM is a bathtub chassis'd shaft driven touring car, so it's going to have more than a passing resemblance to the TC3. Many people nickname it the TC4 and I can see where they're coming from - especailly as AE and Yokomo have a mutual 'back scratching' arrangement going on.
Despite this, no one can dispute the CGM's competitiveness and when you consider that the CGM comes with mainly Carbon Composite parts at a very low price, the CGM is a very attractive proposition. The carbon parts are much better than the standard SD moulded parts, believe me - I did a whole lot of whinging about them.
Didn't I Sosidge?!!