no no no, your not seeing what this is about, its not about the game being more demanding, if you dont got a gpu as ppu the game just dosnt look and feel as good, watch the videos of graw with ppu and without, it addes complexity to the game phsyics thus making the game more real.
3 card solution is for ultra high end or for cheap bastages who want to get a cheap card then add 2 more cheap cards insted of getting 1 good card.
crossfire and sli will never be the norm, infact i kind of see them going away in the future being replaced with cards with more cores or a 2nd card that adds more/new fetures.
look at ppu use this way,
your playing a game, it looks pretty sweet as it is, but wen u shoot the walls u only get little texture changes like black dots(holes) when u blow stuff up it looks ok, not much flying debris or shrapnil and no smoke trails off the debris.
so you add/enable a 2nd/3rd ati card as a PPU, all of a sudden the game looks even better, when u shoot the walls you see real damnage fx, stuff flying, when you blow something up debris flys away allover the place firy smoke trails ricichets(things bouncing around) cars rock when stuff blows up neer them, peoples hair/cloths moves more realisticly, treas/plants/grass move like they are real, the whole game feels/looks more realistic.
thats what a ppu brings.
in CF 2 cards can be maxed out with visual fx and details and not have any extra horsepower to spare to run ppu data(think about crysis/fear/farcry/exct at 1920 res for example)
a cpu is BAD for phsyics, read up on why (google around some) an x1k gpu is GOOD/GREAT for it, hence adding a 3rd videocard to cf or a 2nd card to normal systems.
eventuly they will just tag on units spicificly made to do PPU work on the videocard, the 2nd/3rd card will likely just be used to boost that even more, possably even running more parts of the game on a 2nd/3rd gpu.