How can i activate, if it's possible, usb debug mode from fastboot in a 3rd gen moto g? It is basically dad, it's possible from  fastboot mode to activate de usb debug mode, and if it is, how can i?
I was just commenting on another similar question. Is your phone rooted? You can try this post's solutions to see if one of the methods work.

I believe you can flash a custom recovery via fastboot in case it's not present. 
Eg. - 

fastboot flash recovery &lt;&lt;recovery-name.img&gt;&gt;
fastboot reboot-bootloader
Then use volume keys to select recovery, power button to enter.