Yes Ozone only kills 94% bacteria. Refer to your "Water Care Guide" for chemical requirements

The Paper filters need to be hosed regularly and at least once a month soaked in cartridge cleaner. Before putting back in spa hose off thoroughly. 

Check filters are clean, most error 3 reports are caused by low water flow caused by filters that need cleaning, or low water level. Ensure water level is correct height and remove filters, turn spa off and back on to reset controller then run spa briefly, If the Error 3 Report does not come back the filters need cleaning. 

If the water level has been allowed to drop below the filter tops it is possible an air lock in plumbing will require bleeding. Top up water level and run spa briefly after turning power off and on to reset controller, if Error 3 Report reading returns contact your nearest Arcadia spa dealer.

Possibly the UV unit – remove the access panel at the touchpad end and check where the high pitch is coming from.

This is located on the filter facia in the form of a moulded line.