Will Family Sharing work with 2 Macs and a Windows 10 laptop?

iCloud can only be implemented as a backup in an emergency situation. It is not a classic style backup like an external drive. Instead, if have been using it to sync data files and one of your devices dies, the files may still reside in iCloud and can be synced back to a replacement device by signing into iCloud on that device. Unlike a standard backup, it is only used to sync data files such as documents, photos, etc. If a drive dies it won't have a backup of your system files and settings.

As stated by Phil0124, iCloud storage is shared between users, but each user's files are hidden from other users. There is no fixed division. Anybody in the family can add as much stuff as they want until it all gets filled up.

If you're using this on a computer, you'll also be using it in the context of iCloud Drive:

iCloud Drive FAQ -" iCloud Drive lets you securely access all of your documents from your iPhone, iPad, iPod touch, Mac, and Windows PC."

https://support.apple.com/HT201104

What is Family Sharing? - https://support.apple.com/HT201060 - "Family Sharing lets you and up to five other family members [Apple IDs] share access"

Family Sharing works on an Apple ID basis, regardless of the hardware they use as long as it is capable of performing the sharing functions.

To learn how to 'Set up Family Sharing', click here --> https://support.apple.com/HT201088

"To use Family Sharing, you need an Apple ID signed in to iCloud on an iPhone, iPad, or iPod touch with iOS 8 or later, or Mac with OS X Yosemite or later." You can get some shared material with a PC with iCloud for Windows but apparently you need to have an Apple device to set it up ( https://support.apple.com/guide/icloud/use-family-sharing-with-icloud-mm0b5e79e99b/icloud ).

Actually Android should be in the list too since it can participate in Family Sharing of Apple Music.

Use Family Sharing with Apple Music on Android https://support.apple.com/HT212154

Family Sharing will work with a Windows computer as far as sharing the iCloud storage through iCloud for Windows goes.

You cannot use iCloud to backup either Macs or PCs at all. It does not work that way. It will only work to sync files to iCloud from each device, but it will not work as a backup no.

If each computer is using a different Apple ID, then each will have their own separate iCloud accounts that share the general iCloud storage. Each account is isolated from the rest in that case.

If all 3 computers use the same Apple ID for iCloud, then the account, access and all synced files will be accessible from all devices. In this case, Family Sharing would not be used.