BV3, vaguely, is just a product of speed, power, and toughness. Speed is the part I'm least sure about, but I'm putting it off until I can deal with ammo explosions.
If anyone's interested in helping out, try to come up with an algorithm for rating how dangerous a 'Mech's ammo is to itself. "How much less tough am I because this is here?"
BV2 simply counts every explosive ammo slot that CASE can't save you from as -15 points in one part of the formula (-1 for gauss slots).
BV3 treats each slot as a -20% multiplicative reduction to the 'Mech's internal structure, which is a little better but not great.
Both are flawed in that they don't consider how well 'protected' each slot may be within its location. For example, a Griffin's LRM ammo is less dangerous to itself than a Crusader's LRM ammo. A critical hit to a Griffin's right torso has only a 1/3 chance to destroy the 'Mech, while a critical hit to either side torso on a Crusader has a 100% chance to destroy the 'Mech. But both are counted the same at -30 points. Relatedly, a Thunder Hawk-7X's gauss rifles are far more dangerous to itself than the Griffin's LRM ammo, but BV2 counts those torso GRs as only -14 points. BV3 turns them into a 96% reduction in the value of its internal structure.