A 3-down back means two things:
1 - he's capable of picking up the blitz AND catching the ball out of the backfield, and
2 - your starting RB is that much better than the backup
.
Rotating backs means one thing, but can be good or bad:
1 - your top 2 RBs are of similar quality and/or have complementary skills
a) good: we have a really good backup
b) bad: our starting RB isn't all that good