This markdown file explains how to run Ubuntu on your Acer CB3-111-C670 Chromebook through Crouton.
You first need to put your chromebook in developer mode.
To invoke Recovery mode, you hold down the ESC and Refresh (F3) keys and poke the Power button.
To enter Dev-mode you first invoke Recovery, and at the Recovery screen press Ctrl-D (there's no prompt - you have to know to do it). It will ask you to confirm, then reboot into dev-mode.