Pop-up window: "your system is infected with 3 viruses"

It is a scam for certain, generally tied to a browser.

See if you have unknown Profiles.

To remove a configuration profile in macOS:

1. From the Apple menu, select System Preferences....
2. From the View menu in System Preferences, select Profiles.Note:
3. Profiles won't be visible until you have at least one profile installed.
4. Select the profile you want to remove, and then press the - (minus) button. Click Remove to remove the profile.
5. softwater

User level: Level 5 (5,557 points)

Feb 16, 2020 3:04 AM in response to sukwaria

If your browser prefs are locked down, you'll probably find the reason in System Preferences > Profiles. 

Remove all profiles you don't recognize, then run something like DetectX to clear out the persistence agents. 

https://sqwarq.com/detectx/

Disclaimer: DetectX is a utility I develop; you do not need to pay for it to use it to use it. It does not collect your data or require any form of subscription/login.

It IS a scam.

Close that browser window, quit the browser and reboot your Mac.

You might also download MalwareBytes and run a scan of your Mac to disclose any potential bad actors that may have been downloaded.

Here you go. See this thread which discusses your issue:

"Your system is infected with (3) Viruses… - Apple Community