A direct comparison between the sales of third party games does not really give a fair assessment of how well third party publishers are doing on the platforms ...
For the Wii the lower development cost, fewer high quality third party games, lower (average) quality of third party games, and shorter time on the market (as compared to the XBox 360) means that third party publishers are doing very well on the system; you don't have to take my word for it, just look at what most publishers are saying about the Wii, and how they're going to increase development in the future.
The XBox 360 and PS3 greatest strength is that few games are released exclusively for any platform, all platforms are needed to justify the development, and the combined sales of games have been doing very well in general.
