Addressing the Wii part of the question.
As previous answers showed, there are 2 factors in consideration when emulating: the hardware present at the console and the software present at your PC.
A Wii has the following specs (taken from Wikipedia):
- CPU: IBM PowerPC "Broadway" (Single-Core @ 729MHz)
- Graphics: ATI "Hollywood" (@ 243 MHz)
As you can see, in terms of processing power, a modern PC clearly surpasses a Wii. Hence, it is possible to emulate Wii on a PC. But this just opens the possibility. In order for this to become a reality, one needs to write specific software. Why? The reasons are too complex to discuss at Arqade but short story the hardware of both console and PC don't talk the same language, and the emulator ends up being a translator program between your PC and the software made for Wii.
The complexity of such program increases as the complexity of the console increases. As the Wii is comparatively simpler than the XBox or the PS3 systems, there exists some emulators for it.
XBox and PS3, however, are slightly more complex systems. The XBox in particular is based around a standard PC. Microsoft however made several modifications to the underlying systems and issued custom hardware. This, coupled with the difficulty of reverse-engineering all the processes involved and translating them to a PC, makes the existing XBox emulators more of experiments. The reasoning I use here for the original XBox can be expanded to XBox 360 or the PS3.