Hi Amedcary,
Since the AP is an outdoor one this has a hardware limitation to reduce the power to not less than 8 dBm.
The command to do so is below:
# Set the transmit power to 3 dBm for radio 0 of AP 1.
<AC6005> system-view
[AC6005] wlan
[AC6005-wlan-view] ap-id 1
[AC6005-wlan-ap-1] radio 0
[AC6005-wlan-radio-1/0] eirp 3
Info: The EIRP value takes effect only when automatic transmit power selection is disabled, and the value depends on the AP specifications and local laws and regulations.
So even if the customer chooses 3 dBm the actual EIRP will remain 8, the minimum supported for this AP.
Verification below:

Hope to help you!