Where should a strobe (Firehouse Arc V) be placed on the DJI Air 3 to be compliant an minimize impact to center of gravity.

The FAA doesn't regulate the placement of anti-collision lights, but I put them on top and centered since the manned aircraft they are intended to alert will be in the airspace above my drone.  Also, if you put them on the arms the camera can catch the flashing light.

I added two. One on top near the front, and one on the bottom toward the rear close to the battery.

Before placement on the top, consider that the GPS module/antenna is located on the forward end of the drone on the top and that the strobe may impact satellite reception to some extent.