Typically, the flex option in roster lineups allows fantasy owners to use a running back, wide receiver or tight end in a starting spot. If you're serious about winning your league, don't gloss over this position as a bonus.
In leagues with standard scoring settings, a strong running back trio could take you all the way to the postseason. However, it's not recommended to start your draft with three tailbacks in the first three rounds. Take a look at non-starting ball-carriers who may remain available midway through the draft.
In addition to choices at running back, don't forget the wide receivers and tight ends projected to see a high target volume in the passing game.