Recently bought three young goats. Two have died and the third seems to have diarrhea. Was told he may have coxitia. What is this and how is it treated?

You may have had it spelled wrong. I think it should be coddidia.

Coccidiosis is the disease caused by coccidian infection. Coccidiosis is a parasitic disease of the intestinal tract of animals, caused by coccidian protozoa. The disease spreads from one animal to another by contact with infected feces, or ingestion of infected tissue. Diarrhea, which may become bloody in severe cases, is the primary symptom. Most animals infected with coccidia are asymptomatic; however, young or immuno-compromised animals may suffer severe symptoms, including death.

While coccidian organisms can infect a wide variety of animals, including humans, birds, and livestock, they are usually species-specific.

I am not sure if it infects goats or not. Was it diagnosed by a vet ?

According to this site, it's a "stealth killer in goats".

Tennessee Goats