The experiences I have had are similar to Jason's in terms of the effectiveness of two blades on marginal or even rib hits. While I taken plenty of critters with three blades over the years, I have discovered that when I have hit ribs or spine, the two blade seems to penetrate much deeper. This may be less of an issue with bows that are 55# +, but considering I shoot bows that are 47# and less at 27", small differences can become significant in terms of performance.
Of course, as has been mentioned previously, there are plenty of other factors that also effect penetration and I pay attention to them all...high FOC, reasonably heavy arrow, super sharp head, limited shot distance, and shot placement. While the latter factor may not always be completely in control of the archer, each of the former factors are. By incorporating them all into my bowhunting recipe, I maximize my opportunities for a quick, clean kill...and from a hunter's perspective, that's what it's really all about.
Claudia