On Sunday, police received three different 911 calls within three hours from community members reporting being shot at.
Police say the suspects used toy guns that shoot out gel-filled pellets - up to 200 feet per second.
Public Information Officer Jerami Surratt said one of the victims thought he was shot with a real gun.
"So obviously there's mental stress and all that goes a long with being shot with anything," Surratt said.
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Victims were shot in the body and face and three people have been arrested.
"So they're being charged with assault with a deadly weapon they did shoot the victims in the face one of the victims almost got shot in the eye," Surratt said.
The victims told San Mateo Police when they were walking, a white BMW sedan would drive near them and a passenger would shoot at them.
The third incident that was reported on Sunday was off Monte Diablo Avenue, near Martin Luther King Park.
Police say it was near there where a man and his pregnant wife were shot multiple times by the suspects.
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San Mateo resident Laura Williamson was at the playground with her son.
"It's really surprising because I've never seen anything like that I've been here lots of times and it's always felt safe," Williamson said.
Damian Ancukiewicz lives in the neighborhood and read about similar cases involving BB guns on Reddit - he didn't know about this most recent case.
"I had no idea it was here on this block which makes it all the more real," Ancukiewicz said.
Police say this "appears" to be a trend - but they are still investigating.
"The blatant disregard for the safety of others elevated this up to a felony," Surratt said.