Contacts, iCloud, multiple devices

greetings. I have iPhone XS, iMac desktop, and a MacBook Pro running High Sierra. I have about 2,000 contacts. I have not been able to figure out how to keep the same list of contacts on all 3 devices. Sometimes a new contact will update on another device, often it will not. (I confess that I have been adding contacts on all 3 devices.) I have tried to sync them all through iCloud, but have not succeeded. Don't know where to go from here, need someone to walk me through it from the beginning, I suppose. Thx for your help.