PS3 has never been more powerful than the average PC of this year, last year, or even 2 years ago. UT3 cannot keep a fluid gameplay on a few maps due to the PS3 not being powerful enough/not having enough RAM.
In Xbox 360's case is more about lack of storage in their disc format, since they have enough RAM to give that extra mileage for those maps that PS3 could not handle. Though Epic could put use of 360's HDD to install UT3 and, therefore, have more maps available, I understand their decision of leaving that out as many people do not have HDD for their 360's. Besides, not many people will notice the disappearance of a few maps.
Again, it's more of a RAM issue from PS3, and for the 360 it's more of a non-use of HDD because if the game had required HDD we probably would've been able to see the entire uncut UT3 version on the 360.