iCloud slow to synch across three Macs

Define "slow", slow can be mere seconds, minutes, hours, etc. so we have no idea what you mean. Also, by chance do you have any of the following types of third party apps installed on any of the computers:

• Antivirus
• Cleaning
• Security
• Maintenance
• VPN

Okay, your speeds are not horrible. Depending on many conditions such as amount of data, time of day, etc iCloud syncing can be almost instantaneous or overnight.

Also, the reason I asked if you had ANY of those types of third party apps is they are ALL bad news and should never be installed on a Mac. In order to keep Mac OS secure, fast and working correctly simply keep it up-to-date and restart the computers about 1x per week. Those types of apps do NOTHING that Mac OS does not already do and in most cases have the exact opposite effects that users want. They make Mac OS slow, unstable, appear buggy and cause very unusual behaviors. So, if you don't use any (antivirus, cleaning, security, VPN, or maintenance) apps on your Mac PLEASE leave them that way and keep Mac OS up-to-date and restart as I mentioned earlier. Also, if you use third party apps, it is wise to check them several times a year to ensure they are also up-to-date.

Also, ensure you backup each of the iMacs at least 1x per week using Time Machine if you are not already. If you do not, that is a 100% guarantee you will lose some data. Remember, iCloud is a syncing service it is NOT a backup app.