Asshole: Someone who is rude, selfish, or both. Basically a stronger version of jerk.

Jackass: Someone who is careless or stupid but not rude or selfish. I never call anyone a jackass, but I almost exclusively see it as a term used to deride someone you’re familiar or friendly with for being mischievous or careless.

Dickhead: Someone who is careless or stupid as well as rude or selfish. All dickheads are assholes, but not all asshole are dickheads.