Whilst I'm sure it's a Pseudopimelodus, any chance of some other pics? I know these can be aggressive, but I'm surprised it has been so aggressive.
I'm not sure if it's possible after three years, but can I suggest you try and wean it off live food onto shrimp/prawns/chunks of freshwater fish (trout etc) - it will take some time, but I think it will definitely be better for the fish's health in the long run. Plus there is also the chance you will introduce parasites/disease via the feeders, and from an ethical point of view, many will argue it is wrong to feed fish this way. In the UK, aquatic retailers can get in trouble with the law for knowingly selling fish that will be used as feeder fish, though not sure how this applies in the US.
Anyway, one last question - how often do you feed it? Any predator like this will feed on an irregular basis, so I'd suggest only feeding once or twice a week, and varying the diet as much as possible. You could try tying a prawn/shrimp to a piece of cotton and teasing the fish with it to get a reaction, though, as i said, after three years it's not going to be an easy transition.
Still, a lovely fish, wish i had the room for one.
Martin