this is a mild read and might get complicated so please read this through.

firstly: how do you nullify the triple stabs.
this mostly depends on the stance the peacekeeper has, or the action she did before the grab.
then once you get grabbed and the stabs come out, you need to guard in the direction the peacekeeper had her stance.
example:
peacekeeper has upper stance before the grab, when she grabs you, you have to flick the guard upwards.

pretty standard right? now onto the more important part of this thread.
guarding the stabs are different between characters with a permanent block, and a temporary block.

the easyest is with characters with a temporary block. these characters just need to flick the guard in the right direction (following the example: up). they are able to guard all 3 stabs
now permanent block characters have it seriously harder, and I do not know if this is intended. as it seems you need to actually change your guarding direction to the right direction. if you already are in the upper stance you are not able to do this.
therefore you would need to tank the first hit, and while that is happening you can change your guarding direction. then when the second stab is comming you can change the guard direction in the correct way to block the second stab. however this makes the 3rd stab absolutely unblockable because you cannot "enter" the right guard direction.

Im not sure if this came out as intended. basically to fix this bug (if it actually is non-intended) could it be possible to change the permanent block into temporary blocks during grabs only ?

I hope this helps