Optimizing penetration on modern crossbows and compounds seems like an arbitrary affair. Most bows today will pass through a deer with just about any broadhead and still bury itself in the dirt. So extra blades don't hurt and sometimes they're the difference between catching an artery or not.
However on traditional gear a 2 blade head is an absolute must since you're shooting HALF the velocity of even the slowest Compound bows and crossbows.