Investigators say the incident happened around 8:30 p.m. Saturday at a home on San Pablo Avenue near Belmont Avenue.
According to police, the toddler was home with his parents and a family friend that was handling a gun. When it discharged, the boy was shot in the leg.
Community Regional Medical Center staff alerted police after a woman arrived with the 3-year-old gunshot victim.
The boy's injury is non-life-threatening.
Shortly after, the 28-year-old that fired the weapon was hospitalized. Investigators say the boy's father beat the man when he learned his friend's gun was the source of the shooting.
Investigators say the man that fired the weapon was arrested for several charges related to the injury of the child and unlawful gun possession.