a question about the deal damage block

I made 3 enchantments called berserker-deals more damage when you're under half, health bound-lower your health higher the damage,backstab-deals more damage when hitting enemies from behind.

my question is, do these damage stack or do I not understand Minecraft damage. the problem is 9 damage x 3 from berserker x 3 from backstab and + 9 from health bound = 90 damage but it doesn't 2 shot an iron golem instead 3/4 shot an iron golem even though iron golems only have 100 health/50 hearts