It’s an exact port of the original PS2 with a face lift and mulitplayer support:
http://www.armoredcore.net/ac3_en
In Japan AC3: Silent Line was just released for PSP in Novemeber, and AC3: Last Raven has been announced for 2010. Digital download only I believe, as I haven’t seen it for sale on Gamestop or Amazon - but works for both the PSP and PSP GO.
However it was learned that the ESRB had rated AC3: Silent Line Portable – which means From Software has a finished version of an English port on the wings… and the sudden “price drop deal” for AC3 Portable, makes it seem the release date for the West is getting close. Data from AC3 Portable, can be ported into AC3: Silent Line Portable.
IMHO the AC3 generation was the best - even better than AC4 - although it’s probably because AC3 was when I actually did competative play with people.