I'll second what frickenjoey said; The three speed fits in the case with the reverse module. You may have to do some shaving of the rubber adjustment hole plug, but it does fit.
However, and this is the important thing to know; If you're going to install one, you might as well install both, as they both require a complete tear-down of the trans case. The three speed uses a different final gear, and the reverse module replaces the shaft that the final gear rides on (the one that the outdrive cups attach to).
Also, if you don't already have one, you'll need another servo for the reverse module. A standard servo, such as the airtronics 94102 works perfectly, as speed and torque are not dead-on requirements here. The throttle servo from the savage actually works pretty good. Did I mention I have a servo for sale yet? (shameless plaidfish-style plug)
I would also recommend getting a three channel radio over the mixer. The mixer throws the truck into reverse after so-long of holding brakes. And if you don't come to a complete stop and keep the truck still during direction changes, you run the risk of breaking the sub-gear train. And yes, the module is all gears, no clutches, like Traxxas's setups.
If you don't already have it, the X center transmission case is your friend. To adjust the 1-2 shift point, you have to partially disassemble the multi-speed assembly. Since yours is the X, obviously you have it (this info inserted for the people that search for info, just in case).
Lastly, when it comes time to tear the truck down, there are some tricks to get the transmission out easier than the papers show. Simply remove the tank, pipe, roll cage, tires on the pipe side, the pipe-side upper shock ends on the inner two shocks, and the tvp on the pipe side. Then a few screws on the tank side and the two that mate the trans case to the engine plate, the brake linkage, and then give it a tug. The trans case will be a bit of a wrestling match to remove, but it will come out. Reverse process to re-install.
It's easier than it sounds, really.