Misfires on the Mazda6 3.0L are typically caused by bad ignition coils. You can try swapping the coils and plugs to other cylinders to see if the misfire moves. If it does then replace the bad coil AND plug. Be sure to mark the cylinder 1 and 3 coils so you can keep track of their location during the testing.
You can also check them visually by looking for small cracks in the top of the coil where the electrical connector is. If you find a crack then remove the boot from that coil and remove the spring inside. Look at the top of the spring for rust. If you find rust that is a sure sign that water has been leaking through the coil and it will need to be replaced.