Displaying a what3words address is completely free. The easiest way to do this is to create an embed code of the 3 word address here: https://what3words.com/products/embed-code-generator/ and then add that to your site. 

Alternatively you could use a simple hyperlink in the format of ///word.word.word which links to the correct URL on our mapsite.

There are a number of alternative ways of displaying a 3 word address here:  https://what3words.com/how-to-display-a-3-word-address/