In a Broadcasting and Cable piece about the future of broadcast television comes this admission about the standard lowering underway for broadcast television shows:
Executives interviewed for this article admitted that in 2010, a heretofore lowly 3in the 18-49 demo should be considered a general benchmark for success, with a 4 signifying a hit and a 5 a smash hit.
That's definitely not a "one size fits all" statement for the broadcast networks though.
I'm going to guess these unnamed broadcast executives were more heavily weighted towards Fox and ABC, because that statement makes more sense based on their recent records. For both of them a 3 rated show would be slightly above average so far this season (excluding Fox'sbump), so next season calling that a success seems about right.
On the other hand, for NBC, a 3 rating is a hit this season. And for CBS a 3 rating is below average this season, hardly a success (unless the definition of success has been reduced to simple survival), and my guess is with CBS' fairly low year to year erosion, it's unlikely to be much different next year.