Lacy is a very good receiver and pass blocker. Starks has stone-hands tendencies if the ball is not right on him, but his pass blocking has improved significantly. Both can pound it between the tackles, of course, and Starks is fairly quick to the edge if there's a matchup to be exploited there. In other words, there's not much the 1 and 2 don't have covered.
Harris is a decent all-around back. We've seen him run and catch effectively. He's a surprisingly good blocker for a short guy, probably better than Starks. It's hard to think Lacy and Starks will both make it to the end of the season without one of them missing some time. At that point, Harris will get some snaps.