The deeper you get into a draft, the more your strategy will shift. That's because of who you've already drafted. So it's natural that drafts will veer away from rankings as teams look to fill their starting lineups with players from various positions.
Especially in a PPR league, I'm not going to completely hate if you wind up with WRs for your first three selections. It's not a terribly "safe" way to go, but I can see how it could happen.
On the flip side, if you walk out of Round 3 having not yet drafted a RB, you're going to be in trouble. I don't foresee a multitude of scenarios where you wind up with two RBs through three rounds, but I think that you need to try to get at least one RB fairly early. The RB position falls off of a cliff quickly, and if you don't get one of the top backs you will be playing catch-up at the position all year long.
Once the top tiers of QBs and TEs are drafted, expect there to be a massive run on WRs while any remaining high-upside RBs are snatched up. Again, this is probably too early to reach for a player with significant injury/medical risk. There are enough solid options that you should be waiting until at least Round 5 or 6 before even thinking about pulling that trigger.
For reference, I will be listing each player's Expert Consensus Ranking - PPR as determined by Fantasy Pros.