Originally Posted by
Durzlla
he answered your simple question...you're not grasping the answer.
3x Bleeding 5s, at lv 80 bleeding hits for 42.5dmg per second per stack, at 3 stacks it hits for 127.5 dmg per second for 5 seconds, which puts it at 638dmg TOTAL (due to rounding), this is EXACTLY what the tooltip says.
This is also the EXACT same way that Bloodhunter explained it, i don't think it can be broken down anymore...
EDIT: In your opening post it means that over those 5 seconds, at 3 stacks of bleeding you're dealing 84 damage.