[eluser]jedd[/eluser]
MVC doesn't change the algorithm here - you'd do this in your view, and you'd do it by cycling through a counter to trigger a new row, as you suggest, every third item.
I gather you're okay to write that code, yeah?
If you want to get funky, you could make it a code partial view that you call in from your controller in different places, which means generalising the view. In my (humble) experience this can be more trouble than it's worth, though .. depends on the size of your codebase I guess.