throwing a Blind spot in there is helpful as a 4th bleed long as your behind the target its no problem
Just to clear some stuff up for people who don't know much about bleeds.
Bleeds that contribute to Grievous Wound:
Bleed (from Shadowstab)
Bleed (from Blind Spot)
Bleed (from Sneak Attack)
You can only have one of those bleeds active at any time on a target. They are considered the same bleed.
However,
The only ones that contribute to Wound Attack are the following:
Bleed (from Shadowstab)
Bleed (from Sneak Attack)
Grievous Wound (from Low Blow)
The bleed from Warriors (Slash) stacks with everything else but not contribute to Wound Attack.
tl;dr
Wound Attack will only accept Sneak Attack & Shadowstab's bleed to active a 3rd hit.
As far as I can tell from the most recent patches in Chapter 4 broke the bleed of Blind Spot. So it will not work with Wound Attack.