What is being doubled or tripled in a doublebock/triple bock? Some beers are classed as "doublebock" and some as "triple bock".  Doubles seem to be a little stronger (higher APV) than average and triples seem to be stronger than that, but is that they definition or an effect?  What exactly is being doubled or tripled in the production of these beers, and is the primary goal strength or some aspect of flavor or something else?
Doppelbock (or double bock) is intense in its maltiness and higher than "single" bock in terms of alcohol content, typically starting around 6-7% and going up to around 13%.  There isn't anything specifically doubled or tripled; rather, doppel idiomatically refers to be being "bigger"/stronger than a standard bock.  In terms of how higher "maltiness" is accomplished, that would depend on the specific beer; mashing at high temperatures, reducing lautering, using certain grains or different yeasts, and other factors all can increase the maltiness of the final product.

A triple bock would just be a naming convention emphasizing even further the maltiness and alcoholic strength of the brew.

As an aside, doppelbocks are an evolution of strong monastic brews used as "liquid bread" for fasting monks, as they were not allowed to consume solid food.  These sweet, malty beers evolved over time into the modern doppelbock.