This is the available (so called) 3 speed kit for the TMaxx black transmission. I do not recommend it as it fails rather easily and does not increase top speed very much when it does work.
http://www.radrc.com/store/product_info.php?products_id=76
You can modify the stock black transmission to have a true 3<sup>rd</sup> gear with a large increase in top end speed (with a larger displacement high torque engine) but it requires quite a bit of modification and addition of internal gears, alterations to the shafts and the transmission housing. Not an easy mod by any means.
I can post a link if you’re interested.