Hex requires concentration, while bestow curse at a high level and Hexblade’s Curse do not. This allows all three things to stack.
- Hex deals damage whenever an attack hits
- Bestow curse deals damage whenever an attack or spell inflicts damage
- Hexblade’s Curse adds damage to all damage rolls
If all three were applied to a target, for every time I hit it, would it unfold as follows:
Hit, hex damage, bestow curse damage triggered by hit, bestow curse damage triggered by hex, and all 4 damage rolls get boosted by Hexblade’s Curse?
None of the listed effects add an additional damage roll, they all augment the one roll you make.
(all emphasis mine)
So while they do stack, the damage would be:
basic damage + d6 (hex) + d8 (bestow curse) + proficiency bonus (hexblade's curse)