Yeah, I kinda agree you should be more worried about a 3rd RB with the flex position.
What you DO have is two very solid RB's, and a couple nice WR's. Your also set at QB. A 3rd WR is usually about the quality of the guys you have as your 3rd and 4th WR's are.
I'd keep an eye on the Jets situation in the next couple of days. A trade that makes sense to me is Duckett, since ATL certainly appears willing to trade him, and the Jets are actively looking for an upgrade at RB.
Dominick Rhodes might be a good grab (assuming Addai was drafted).
The Denver situation is in it's perpetual toss-up stage. Maybe Mike Bell or Ron Dayne would be a good pick up (in that order).
DeShaun Foster seems to get chronically injured, so maybe DeAngelo Williams should be on your radar as well.
There's almost always a RB who gets hurt early, so be quick to swoop in on one before you hear any official word.
You look pretty set at WR for the opening weeks. I'm hoping Troy Williamson can have a big year for my Vikes, and he doesn't always get drafted. Lots of speed and potential there, and he'll start. More of an obligatory, homer plug than an actual tout for a breakout year, but the kid can do alot when given the ball.