"Your mac is infected with 3 viruses"

Did the application ever open and run? You may  have adware/malware installed. Try downloading this program which was written by Thomas R, a long time poster. The program will search for malware/adware The program will do the work for you which makes it easy. It may be necessary to run more than one scan. After running the scans, restart the computer.

Malwarebytes Anti-Malware for Mac      10.10 and later

Malwarebytes uninstall

If you start having problems, try running this program in your normal user account, then copy and paste the output in a reply. The program was created by etresoft, a frequent contributor.  Please use copy and paste as screen shots can be hard to read. Click “Share Report” button in the toolbar, select “Copy Report” and then paste into a reply. This will show what is running on your computer. No personal information is shown. If the log won’t post, try posting it in Pastebin and provide a link in a reply        Pastebin

Etrecheck – System Information    10.10 and later

This is a scam. Ignore it. Stay away from sites with unknown links.

These are scams and should be ignored

1. Identify and report phishing emails and other suspicious messages
2. Annual Reminder- Watch out for email Phishing Scams
3. Identifying fraudulent "phishing" email
4. Beware of Browser Pop-Up Tech Support, Phishing Scams
5. Phishing & Other Suspicious Emails
6. Avoid phishing emails, fake 'virus' alerts, phony support calls, & other scams
7. Phony "tech support" - "ransomware" popups and ... | Communities

Report phishing email to abuse@apple.com.

Fixes for Adware and Pop-ups

1. Malwarebytes
2. DetectX Swift
3. Remove adware that displays pop-up ads and graphics on your Mac
4. Stop pop-up ads and adware in Safari - Apple Support

If you find that any of these pop-ups prevent access to Safari:

Fixing Safari from Popups 

[The following comes from user stevejobsfan0123. I have made minor changes to adapt to this presentation.]

Fix Some Browser Pop-ups That Take Over Safari

Common pop-ups include a message saying the government has seized your computer and you must pay to have it released (often called "Moneypak"), or a phoney message saying that your computer has been infected, and you need to call a tech support number (sometimes claiming to be Apple) to get it resolved. First, understand that these pop-ups are not caused by a virus and your computer has not been affected. This "hijack" is limited to your web browser. Also, understand that these messages are scams, so do not pay any money, call the listed number, or provide any personal information. This article will outline the solution to dismiss the pop-up.

Quit Safari

Usually, these pop-ups will not go away by either clicking "OK" or "Cancel." Furthermore, several menus in the menu bar may become disabled and show in gray, including the option to quit Safari. You will likely have to force quit Safari. To do this, press Command + option + ESC, select Safari, and press Force Quit.

Relaunch Safari

If you relaunch Safari, the page will reopen. To prevent this from happening, hold down the 'Shift' key while opening Safari. This will prevent windows from the last time Safari was running from reopening. 

This will not work in all cases. The shift key must be held at the right time, and in some cases, even if done correctly, the window reappears. In these circumstances, after force quitting Safari, turn off Wi-Fi or disconnect Ethernet, depending on how you connect to the Internet. Then relaunch Safari normally. It will try to reload the malicious web page, but without a connection, it won't be able to. Navigate away from that page by entering a different URL, i.e. www.apple.com, and trying to load it. Now you can reconnect to the Internet, and the page you entered will appear rather than the malicious one.

how to quit safari

Safari menu/Quit Safari or try command - option/alt - esc to Force quit.

You are welcome.