The Rocket uses OBD II technology and you can pull codes, clear codes, and view certain live data with a generic scanner...but that's about it. This info can be useful depending on what you are doing.
The Triumph scanner can help do things like set up primary throttle potentiometers, it has functions built into it to override the stepper motor when doing so... it also is used to download Triumph factory tunes and so on...
The PC III really has very little use as a diagnostic tool, it enhances the factory tunes for better performance or to change fuel maps for aftermarket equipment ect.
The Tuneboy does have some diagnostic functions... but boy, are they slow (at least on the laptop we are using
). Also, you can change fuel maps, timing, and so on ... play with A/F ratios, turn off the secondary throttle plates, correct speedo error.. remove the MPH and REV limiter... but you really need to know what you are doing to use it. Somewhat of a learning curve on this...
So it really depends on what your goal is... if you are buying aftermarket exhaust and just want it to run at its best and don't have the skills to re map it... the PC III would be the way to go.
If you are doing some custom performance mods (such as a turbo) and have a dyno and time and knowledge the Tuneboy would be great...
If your bike has a problem and you are a do it yourselfer you might want to play with a generic scan tool...
If you just want a differnt Triumph tune or want to get your throttle bodies synched... you'll have to pay a Triumph dealer...