Construct 3's Developer Mode enables extra testing features for developers, such as a special addon testing mode.
To enable Developer Mode, open Construct 3's settings dialog and click or tap the dialog caption 10 times. A prompt will appear asking if you want to show developer mode settings. Click OK.
Now in the settings dialog there should be a checkbox named Enable developer mode. Tick the checkbox, close the settings dialog and restart Construct 3. You are now using Developer Mode.