All three victims were transported to a local hospital. A woman driving the bus had moderate injuries, while the man driving the big rig was in serious condition. A man in a black Lexus sedan received minor injuries.
The cab of the big rig became separated from the trailer and tumbled down an embankment onto the Sierra Madre Boulevard onramp to the westbound freeway, Derderian said. The trailer was left on its side.
The cause of the accident is under investigation.
The bus is a Metro Orange Line bus that the MTA contracts out to Southland Transit.
City News Service contributed to this story.