This article will help with enabling What3Words for your account how it works
To set up What3Words in your account. Go to Settings (Cog top right of the home page) from the home page and then into General Settings.
Under the General tab a little way down, will be a switch where you can enable What3Words for your account.
Once enabled, when you go to create or edit a contact, you can enter the What3Words for that customer's address. (This can also be added for site addresses when adding or editing them)
When entered and saved, there will be a button on the contact record, where you can look up your customer's location through What3Words.
There will also be a What3Words button on appointments for customers that have it entered to help with finding your customer's address.