My apologies.  I did not see your comments before making the most recent edit to the article.  I have no problem with retaining the "big three" concept.  The sentence in question just needs to be written a little better.  We should eliminate the words "big three" and replace them with something more descriptive and less simplistic.  I like your wording, "...three design elements...firepower, protection, and mobility." Perhaps this could be used to replace "big three" with some additional wording.  How about, The three principal design elements ....?"  For the record, reliability refers to the entire weapon system, not just how well the tank gets around the battlefield.  For example, if the main gun sights don't maintain proper alignment, you can't hit anything.  Reliability right? You can have the best tank in the world in terms of battlefield mobility, but if it keeps throwing tracks, it's unreliable.  Anyway, a rewrite of the sentence in question will almost certainly solve our problem.