Adenhart, 22, and two of his friends, 20-year-old Courtney Stewart and 25-year-old Henry Pearson, were killed. Another friend suffered critical injuries.
In the videotaped interview shown in court, Gallo said he drank beer and did shots of sake. He later said he didn't remember how the accident happened and expressed remorse.
He has pleaded not guilty.
If convicted, Gallo faces a maximum sentence of 54 years and eight months to life.
The Associated Press contributed to this report.