How does debuff/buff stacking work?

If poisons have a stacking 30% regen reduction why doesnt 3 poisons cancel healing?

Comments

3*30=90%

Even with over 3 poison, champs can still heal

3*30=90%

Even with over 3 poison, champs can still heal
3*30=90%

Which champs? Are you taking abilities and aar into consideration?

Poison reduction does not stack on base. It stacks on current regen. So 30% of 30% of 30% of 100%.
So you're left with around 34.3% healing