How do I protect my iMac, iPad, and iPhone from viruses?

All of the protection you need is built into the operating system.

Do NOT install any 3rd party antivirus software. It is incapable of providing the protection it advertises.

Not to mention the fact that there are no known viruses in the wild that can infect a Mac, iPad, or iPhone that has not been jailbroken.

The best protection and also the biggest risk to your devices is your understanding andbehaviour. Apple devices are quite hard to attack from a technical point of view - so the user becomes the weakest link and therefore the easiest point of attack. There have been a couple of posts here in the last few days where people responded to an email or text, got sent to a website to enter their username and password to prevent something bad happening (or so they were told) and very quickly lost access to device and AppleID. I get a dozen emails a day telling me that my McAfee subscription is about to expire and my Mac is in danger (I haven't got a McAfee sub.) or that my iCloud data is about to be lost if I don't pay for it (I don't have a paid subscription to iCloud). They look like stupid scams because they are, but with AI they will get better and if they send out ten million mails and texts and only 1 in a million people respond then that's 10 people potentially compromised.

The best thing you can do is stay informed about the sort of scams there are out there. Read up on how to protect your devices using Apple Features like Find My and Lost Device. Use strong passwords - Safari will suggest and save them for web logins and don't re-use passwords for different logins. Use two-factor authorization where you can (research 2FA if you don't know what it is). Don't be panicked into opening links in mails and SMSs from people you don't know and if you get strange messages from people you do know then check with them first because mails can be sent to look like they are from people you know when they are from scammers.

And finally - backup your files, videos, photos to removeable drives that you can store separate from your devices. Google "321 backup" and learn how Time Machine works if you've got a Mac. iCloud is useful but it's a synch service, not a backup.

Reading that back it sounds a bit daunting - scary even. It's not really. The more you learn about protecting your stuff - files, logins, devices, - the safer you'll be and the more at ease you'll be.

+1 Viruses are not the problem.

Apple Devices already have several layers of built-in Protection.

Have a look at > Apple Platform Security - Apple Support

The real problem comes from users downloading bad third-party software, malware and/or falling for phony support tricks.

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

To add my spiel to KiltedTim's warnings:

First, there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This documents describe what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community and Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is designed solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

Unless you're using a true VPN tunnel, such as between you and your employer's, school's or bank's servers, they provide false security from a privacy standpoint.  Read these two articles: Public VPN's are anything but private and Security Risks: The Dangers of Using Free VPNs (eccu.edu).  

Additionally, a new study ("Apple Offers Apps With Ties to Chinese Military”) is specifically about VPN apps in Apple’s App Store.

The new study above proves the point of "just because it's in the App Store doesn't mean it's wise to download and use it".

If you have a concern about an app you are thinking of using just come back and ask.

Thank you for responding with your concerns, Coffeelover75.

MacOS, iPadOS and iOS are the most secure operating systems on the market. Privacy is at the very core of all Apple products and services.

I would advise against installing anything that interferes with that.

Don’t be fooled by clever marketing advertisement tactics that rely on fear to sell you a solution to a problem that doesn’t exist.

All the best! 👋🏼😉

In addition to the excellent recommendations of my colleagues, I suggest you read this:

Effective defenses against malware and other threats

Speak of the Devil. I had this in my mailbox just now:

Oh my! The Cloud is falling. The Cloud is falling. I must hurry up and send in my money.