If a shield implements the IOREF signal that they should be compatible.
After looking at the schematics I'm pretty sure the WiFi and Enet shields are compatible and I think Massimo has said as such, not sure about the motor shield. The control signals go directly to the L298 chip, so the next thing to do is google for a data sheet for that chip, just to answer a simple question like this.
I searched all three product pages and the is not a single mention of Due or IOREF.
EDIT: On the motor board A2 and A3 go to connectors with 5v on one pin, this is a potential problem depending on what you plug into those connectors.
Rob