Separates with a qualification: a good AVR is a good pre-pro. The whole purpose of a three-channel amp as contemplated by the OP is to use it to power the front soundstage (L/R/C). An AVR would be necessary to power the surround channels.
The other point I'd make is, in terms of features and value, you might be hard pressed to find a true pre-amp that's a better bet than a good AVR.
The other point I'd make is, in terms of features and value, you might be hard pressed to find a true pre-amp that's a better bet than a good AVR.