Let's break down [+]³.
+ (the is the unicode mathematical italic small h character, \u+1D455(D835+DC55), a specific math glyph) means Human-plus, the next version of man. In the game that means cyborgs.
The ³ is likely for Human Revolution being the third game in the series.
Taken together [+]³ may mean "next-human cubed", as in an exponential advance in humanity as opposed to an iterative advance.