N508BH, the Cessna, is probably the one they're using for surveillance (See the 2012 OC Register article, "$2.2 million Cessna will help fight crime"[2]).

FlightAware shows that N508BH flies to northern california a lot. You can also see what looks like a probable surveillance flight path at http://flightaware.com/live/flight/N508BH/history/20151128/2...

Three aircraft for a California city with population 350K isn't crazy--Glendale and Burbank together have about 300K people, and their police forces have gone in together to create a "joint air support division" that has 4 helicopters[3].

  [1] http://www.faa.gov/licenses_certificates/aircraft_certification/aircraft_registry/releasable_aircraft_download/
  [2] http://www.ocregister.com/articles/air-356706-police-cessna.html
  [3] http://www.mdhelicopters.com/news/pdf/2014/111914.pdf

As a point of contrast, the New Zealand police own 1 helicopter, and contract out flying light aircraft to commercial operators.

Upon saying that, they often call upon the air force for logistical support (in particular for helicopter operations, usually for drug operations, such as airlifting cannabis for destruction, not for raids).

Helicopter ownership by police departments is common. Airplane ownership is less common, at least at the city/county level - the state police forces frequently have small planes.

I will say, being from the US, the fact that any branch of the government would use such a device should amaze me, but unfortunately I have to admit I have heard of worse abuses of power.