I think FantasyMan13 has it right...at least for the first two rounds. Those top WR's are going to have the equivalent stats of a top RB with the PPR added in. As far as the 3rd round goes, you may want to go WR again if a high possession guy like Housh or Wayne are still around since they should be more predictable than some of the #2 RB's for this year.
Don't worry...even in non-PPR's people are taking top WR's in the second round. It's not a stretch by any means since there are is so much volatility in the "non-stud" RB area. Many drafters are starting a more "sure fire" drafting approach by taking WR's they are 95% confident will perform at a high level in the second round.
I still feel it's easier to find a servicable WR on the wire than it is to find a RB, so if there's an RB you really like that's around in round 3, I wouldn't hesitate to pick him up.