How early to take a 3rd RB?

[tman570]

I try to fill out my weekly linup but if there is a great RB that for some reason everyone missed im all over it. For example, in this years draft (keeper) i have green, westbrook, and owens, with the 4th round most of the good rbs will be gone so I will get a great 2nd WR, if for some reason a good back is left I might snag him instead.