RPI3 Disable LEDs
To disable the LEDs on a Raspberry Pi 3, add the following to the /boot/config.txt file:
dtparam=act_led_trigger=none
dtparam=act_led_activelow=off
dtparam=pwr_led_trigger=none
dtparam=pwr_led_activelow=off
More information: Raspberry Pi Guide - 5.2 Control LEDs