So, a couple of thoughts.
The myTouch 4G has been out for nearly 2 years. From what I can tell, the accessories market for this phone is drying up, not expanding. I truly think that we've seen all the accessories, from HTC and from third parties, that we're likely ever to see, unfortunately, as manufacturers have since turned their focus to the newer and better devices. So, I think what's out there is what you're going to find.
And... no... I have not seen any docks that use the 3 pins. Except for a couple of homemade units, and a product out of China that looked promising for about 4 months until it turned out to be vaporware.
Why there weren't any commercial docks out that utilized the 3 pins, no one can say. As you say, that's what those pins were designed for. Yet, all the myTouch-specific docks still used the micro-USB plug. It is all sorts of frustrating... and baffling. Given that the USB plug tends to get munged in the process.
That said, there are sound reasons why the 3 pins aren't an ideal solution. Literally. Sound reasons. And Sync reasons. The 3 pins are incapable of supporting either, in and of themselves. They are solely charging pins (and the third pin is the trigger to flip the device into dock mode). In order to facilitate audio hookup, you'd either need a hardwire cable (inconvenient), or a bluetooth connection (horrible quality). For sync'ing, you'd, again, need bluetooth (or just rely on the same OTA sync'ing that you use when you're mobile, of course).
Still, a dock, which uses the 3-pins, that I can have on my nightstand to keep my phone charged while I'm home/asleep, would be a welcome accessory. That, coupled with one of the apps out now that act as a wi-fi remote for a media center, and I'd be all set.
