From what I can see third parties have made up their minds after the experience of the last few years in that they gain the most from making multi platform games for the 360/PS3. Sales are obviously affected by multiple factors but primarily I would say it is demographics of the consoles as well as the hardware specs.
Because the PS3 and 360 are comparable in performance, all a developer has to do is make a suitable engine which will be more or less equal across the two platforms. Unfortunately due to it having lower specs the aforementioned engine is unusable for the wii and a seperate engine must be created thus adding to development costs.
The third party company then has to figure out if the added costs can be made up for with the sales on that platform.
If regular gamers have more than one console in most cases it appears they go for the game which is more impressive graphically and that favors the 360/ps3 versions when it comes to sales.
Regarding demographics it has been regularly acknowledged that Nintendo has captured the majority of the casual market and while that is great for hardware sales it can make it tougher to sell 3rd party hardcore games. Casual games and Nintendo first party games have done very well though which suggests that most of the user-base are casuals and gamers who primarily purchase wii exclusives and ignore most multi platform games.
So basically to your question I would say yes but it seems that the Wii is affected more due to the choices that gamers have in the 360 and PS3 consoles.
I guess there is no easy answer but those are my 2 cents. Looking forward to other insights.
