By default, Screencastify will count down from three (both visually and audibly) before every recording begins. This is designed to help you prepare for your recording and provide confirmation that your recording has successfully started.
Change or turn off the countdown
You can shorten, lengthen, or turn off the countdown from the Recording Control Panel.
Open the extension's Recording Control Panel
Click Show more options
To change the length of the countdown, click the number of seconds and select a new number
To turn the countdown off, turn off the toggle next to "Countdown"