H6 is misfiring on 1, 3 and 5

While driving the car lost power and began misfiring on 3 cyliners. No bang, boom or pop. Took to dealer and private mechanic, both of which obtained the same codes PO301, 303, and 305. Dealership wanted 3-5K to fix, though didnt know cause of the problem. From what ive read, could be plugs, wires, coil packs, O2 sensor, injectors, cam actuator and or sensors, carbon on valves? Timing belt? Have good compression on valves. Going to start with plugs and wires. And go from there.