Playing in a League with a Flex WR/RB option

[BriDog]

So I'm playing FF for the 2nd time. My first time was 2 RB and 3 WR, but this year I'm playing in a league where we have 2 of each and a flex. The way my draft went I wound up with 2 starting rb's and 3 stud receivers. Am I putting myself at a disadvantage starting 2rb and 3 wr? I don't really have a great 3rd rb option, should I look to move one of my WRs for a 3rd RB?

This is my roster:

QB -Rivers/Hasselback
RB - LT2, Deuce McCallister, Ladell Betts, Warrick Dunn
WR - Chad Johnson, Javon Walker, Colston, Isaac Bruce
TE - Daniel Grahm, Heath Miller
D - Panthers
K - Stover

[Atog]

I think in most cases you want to use 3 RBs when you can, if presented with the option.
However, I think that you should be fine to start the season here. Ocho Cinco, Walker, and Colston should be a good enough trio while LT and Deuce are decent RBs. Since it seems like everybody in Washington loves Betts, he could be a very effective 3rd RB at some point. Bruce and Dunn are good too.