I've got a simple question. Does Raspberry Pi 2 Model B or the B+ case work with Raspberry Pi 3?
Yes more or less.
If it's transparent, you're good to go.
If not, the case might block the LED lights since it got moved over near the MicroUSB power port.
If it's metallic (fully or partly), it might interfere with the wireless antenna (the antenna is located where the LED lights used to be) and might also block the LEDs.
Depends on your case. If your case doesn't have anything fitting around the core chips, it should work just fine. Otherwise, I'd doubt it. It shouldn't have any issues with the Pi 2 case unless it is fitted perfectly around the core components.
Also, if you can provide a link to a 3D Model or one online, that would give a solid answer.