there's lots of things this could be and it's really hard to know without seeing more info about the computer. if it's 3 seconds and your computer isn't giving you any errors, theres a chance that it's completely harmless. it could be display drivers updating, an external display turning on/off, etc.
From Google:
If your HP Chromebook screen keeps flickering and the usual troubleshooting steps like pressing the power, refresh, and esc buttons do not work, you can try the following solutions:
Check for updates: Sometimes, a software update can fix issues with your Chromebook's performance. Go to "Settings" > "About Chrome OS" and check for any available updates.
Clear cache and cookies: Your Chromebook's cache and cookies can cause issues like screen flickering. Go to "Settings" > "Privacy and Security" > "Clear browsing data" and select "Cached images and files" and "Cookies and other site data". Then, click "Clear data".
Disable hardware acceleration: Hardware acceleration can cause issues with your Chromebook's performance, including screen flickering. Go to "Settings" > "Advanced" > "System" and disable the "Use hardware acceleration when available" option.
Reset your Chromebook: If none of the above solutions work, you can try resetting your Chromebook to its factory settings. Go to "Settings" > "Advanced" > "Reset settings" and select "Powerwash". This will erase all your data and restore your Chromebook to its original settings.
If none of these solutions work, it might be a hardware issue, and you may need to contact HP support for further assistance.