Everyone is right. It completely depends on the flow of your particular draft. Personally, I wouldn't have a problem of ending up with 1RB, 1WR, and 1TE (well, only one) but would be equally satisfied with 3RB's, 2RB's and a QB, or 1RB & 2 WR's. It really just depends on who is available and where you think people are panicking and who they're going to grab on the wrap around.
So... is it ok to grab 3RB's? Yes. Do you have to? No. And that's about as specific as it gets.