Your definition of a third down back doesn't match what basically anyone else is talking about when they talk about a third-down back. If you wanted to have your own definition, I was just going to let it go.
A third down back is a quick, generally smaller back who excels at playing in space and catching the ball out of the backfield.
A quick Google search turned up this article at the top of the list. It's from a couple years ago.
Several of those guys are every down backs. Foster, Law Firm, Gore, Jackson, Ridley, Richardson, Peterson and Martin were all the featured back for their teams. Thus the guys who were more "third-down backs" were Bradshaw (5-10, 217) and Murray (6-0, 217) who obviously turned into a feature back.
That's probably not particularly helpful but here is the run/pass ratio for all teams for 2012:
Even Tebow's Broncos threw more than they ran on third down by a noticeable margin.
Finally, catches by RBs on third down:
Again, several featured backs (Rice, Foster, McFadden, Bush, Forte and Spiller). But the third down specialists were usually smaller guys: Brown (6-0, 223), Rodgers (5-6, 196), Gerhart (6-0, 231), Sproles (5-6, 190) and Woodhead (5-8, 200).
So out of all those guys, only one is within 15 pounds of how big Cross is. And I'd say they all play in space better than Cross (but that's my opinion).