To turn off LEDs on RasPi 3B+

sudo nano /boot/config.txt

Add these lines to the end of the file:

# Turn off Power LED
dtparam=pwr_led_trigger=default-on
dtparam=pwr_led_activelow=off
# Turn off Activity LED
dtparam=act_led_trigger=none
dtparam=act_led_activelow=off
# Turn off Ethernet ACT LED
dtparam=eth_led0=14
# Turn off Ethernet LNK LED
dtparam=eth_led1=14