Police said the men pretended to run a medical marijuana facility in a commercial building in Fullerton, even going as far as painting a green cross on the wall.
"These suspects had spent some time putting this ruse together, putting the order in and find a building that worked for what they were trying to do," said Sgt. Andrew Goodrich of the Fullerton Police Department.
When the courier arrived from Oakland, one of the suspects, who in a surveillance video appears to have been wearing a security guard uniform, let him in.
Moments later, two suspects approached the courier, one of them armed with a handgun.
When the victim tried to fight back, they pistol-whipped him and took off with about nine pounds of pot and the man's cellphone.
Police say the victim retrieved the other half of his delivery. He was treated and released from the hospital.
The suspects appear to be one male African American and two male Hispanics in their early to mid 20s, average height and build.
Anyone with information is asked to call Fullerton police at (714) 738-6716.