I went back and rewatched the first 2 games, solely focused on the middle linebackers (Hilliard, Borland, and Browning). From the first two games, it seems that Justin Hilliard is far and away the best run defender. Borland being the next best. Browning is clearly the least effective against the run.
All 3 LBs are basically untested in pass coverage through the first 3 games, so not much to evaluate there. Browning definitely looks the most fluid dropping into coverage. There hasn't been enough blitzing to make much of a comparison, either.
Based on these observations, you would think that Hilliard should be rotating in with the first team defense at MLB, at least in obvious running and short yardage situations. Browning should sub in during obvious passing situations and/or be moved to the SLB, which seems like a better fit for his skill set.