I'll join in with wyoDreamer on this one. An eight-week CornishX will dress out about 5# or better. If it dresses out to 3# it was pulled and processed early. There are folks that prefer them that age and that's fine. But it's that last few weeks (beyond the age when yours would have been butchered) that they pile on the breast and thigh meat. In those same weeks, their frame and gut grow some but not at the same rate as the meat. It's that rapid-growth nature of the beast that leads to recommendations for restricted feeding to keep them from developing pulmonary and other problems as they near their 8-week prime feed conversion rate. (Lots of folks have been quite successful feeding them on range or in other ways, but the recommendation I tell of is out there and widely observed.)
What might be interesting for comparison is to buy two birds, one like you had at about 3# and another that dresses about 5. It won't be quite the same comparison to the way you weighed your bird because what I'm proposing will leave the bones in the legs and wings, but , after weighing each whole bird, cut them up into breasts, wings, legs and boneless thighs. Then weigh the remaining carcasses. I have an idea you'll find that the carcass weight is a greater percentage of the weight of the smaller whole bird than it is of the larger.
If meat-per-dollar is your aim, I expect the larger birds will be the more efficient. If your preference, for whatever reason, is for the smaller birds, that will be the price and yield.