Fighter
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 1
Communication Breakdown
ATC Issue All Types; Conflict NMAC
Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance
Aircraft; Human Factors
Aircraft X was VFR in a left downwind after departing Runway XXR.  Aircraft Y was on a Visual Approach and cleared to land on XXL. Aircraft X was VFR orbiting 4 miles east of ZZZ. When Aircraft Y was about 6 mile final at about 1;500 ft Aircraft X was about 1 mile south of ZZZ still in the downwind. Aircraft X then decided to cross over final at 1;500 ft in front of Aircraft Y who at the time was at 1;300 ft.  Aircraft X did not give a reason or a traffic insight call. I Local Controller (LC) immediately issued traffic to Aircraft X about Aircraft Y by saying 'turn left immediately for final traffic at 1;300 ft type aircraft.' I don't believe I got a response. By that time he was clear of Aircraft Y and headed straight towards Aircraft Z. This time I informed Aircraft X to turn left heading 360 immediately to avoid the traffic. Again no response. I issued traffic to both aircraft until Aircraft Z reported the Aircraft X in sight and it was no longer a factor. I terminated control of Aircraft X at the edge of the Class Delta east of the field. Aircraft Y and Aircraft Z landed with no further incident.Aircraft X should have informed me of his intentions much sooner. The reason he was LDW [Left Downwind] instead of RDW [Right Downwind] was because of Aircraft Z to the East and IFR final traffic inbound. There should be a set procedure to remove conflict with IFR Air Carrier traffic.
Tower Controller reported an NMAC due to an aircraft turning in front of an arrival.