Shortly before 11 a.m. Pleasant Hill police were called to the complex, located 141 Golf Club Road, on a report of a deceased male in an apartment, police said.
When officers arrived, they determined that the man's death was suspicious and smelled a strong chemical odor in the apartment.
Officers called the Contra Costa County Fire Protection District's hazardous materials team and nearby residents were told to shelter in place, police said.
Fire Marshal Richard Carpenter said three people were transported to Contra Costa Regional Medical Center in Martinez as a precaution. They were having trouble breathing but were not believed to be in serious danger.
Hazardous materials specialists identified the chemical as hydrogen sulfite, which is not believed to be harmful, Carpenter said.
Crews ventilated the apartment and lifted the shelter-in-place order at about 2 p.m., Carpenter said.
The name of the man found dead in the apartment has not been released because his family has not been notified of his death, police said. The case is being investigated as a suspicious death.