Insert the Raspbian MicroSD in your PC and open its content.
Using an ASCII editor create a file called wpa_supplicant.conf with this contents:
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1
country=IT
network={
ssid="your_SSID"
psk="your_PSK"
key_mgmt=WPA-PSK
}
Replace IT with your ISO country code, your_SSID with the name of your wireless access point name and your_PSK with your WiFi password.
Save and reboot your CM3 device with this MicroSD. If you need to change the WiFi access repeat this procedure. Note that after the first boot this file will be deleted.