I only manage to brew a poison with 3000 damage ( Human Heart, Crimson Nirnroot, Jarrin Root ), but how can others brew poison with 7000+ damage or even 9000+ damage?

are you using enchanted alchemy boosted gear? also you might try one of the fish, I think its cyrodillic spadetail, or River betty. I think river betty gives me a 75 point (non jarrin root) conventional poison when the other ingredients only give my 30-45 points of damage.