Final answer:
In a three-party accident stemming from an illegal alcohol sale, the involved parties are the operation serving the alcohol, the injured person, and the seller of alcohol. Law enforcement and insurance companies play roles but are not direct parties.
Step-by-step explanation:
When a three-party accident results from an illegal sale of alcohol, the parties involved are the establishment serving the alcohol, the injured person, and the seller of alcohol.
These parties may include the operation like a bar or a restaurant, the individual who suffered harm, and the individual or entity that illegally sold or provided the alcohol. The establishment serving the alcohol could be held responsible for negligence under dram shop laws, depending on the jurisdiction.
It is pertinent to note that law enforcement agencies may get involved to investigate or take action regarding the illegal sale, and insurance companies may be involved regarding claims related to the accident, but they are not considered direct parties to the accident in the context of the question.