Open /boot/config.txt and add to the end of the file:
# disable wifi
dtoverlay=pi3-disable-wifi
# disable bt
dtoverlay=pi3-disable-bt
Reboot!
Alternatively you can blacklist kernel modules for wifi and/or bluetooth
Create /etc/modprobe.d/raspi-blacklist.conf and copy this:
## wifi
blacklist brcmfmac
blacklist brcmutil
## bt
blacklist btbcm
blacklist hci_uart
Reboot!