Zombicide Combat Rule - Multiple Dice
From the rules for melee combat,

Each die roll equal or higher than the Accuracy value on the
weapon’s card is a successful hit. The player divides his hits as
he wishes among the possible targets in the Zone.

This means that a melee attack landing three hits could target and kill up to three creatures.
Ranged combat works the same way, except there are limitations in the order in which you can target monsters (targeting priority).
Keep in mind that there's no point in assigning more than one hit per creature.

Fatties are killed with a 2 Damage hit (or more). It does not matter how many hits you obtain with a weapon that inflicts 1 Damage.  A Fatty will absorb the hits without flinching.
Abominations are killed with a 3 Damage hit (or more). As no weapon naturally has Damage 3 in Zombicide: Black Plague, you have to destroy the monster with Dragon Fire (see page 35) or with Samson using a Damage 2 Melee weapon in conjunction with his +1 Damage: Melee Skill.