Cannot have more than one target selector argument

I expected all three Creepers, Zombies and Skeletons to die when a command block got activated with the command "kill @e[type=Creeper,type=Skeleton,type=Zombie]"

But only the Zombie died

Steps to Reproduce:
1. spawn some different type of mobs in a loaded chunk
2. type in a command "/kill @e[type=mob1,type=mob2,type=mob3 etc.]"
3. Only the last type of mob in the command ex:'mob3' gets killed