I have the what 3 words app on my phone , as a back up, and it might prove useful in unusual situations.
But I cant really see that Garmin would agree to any integration - unless they buy the company. But Garmin seem to be less interested in global positioning technology these days - everyone and their dog can do it.
What 3 words seems quite good for finding a precise location that is not obviously identifiable by a road or the address,such as an obscure doorway on an industrial estate or somewhere rural. (IIRC the Authorities in Mongolia use 'what 3 words' as their official post code .- Is it racial stereotyping to say it would be good for locating itinerant yurts on the steppes? )
Yes you can do the same with google maps etc using map co ordinates, but if I ever I need to call emergency services under stressful circumstances (or simply direct couriers etc to my location) it will be easier using 'just 3 words' than having to relay a string of numbers.
When checking some of the squares on my driveway and garden I have found quite a few where the 3 words are quite amusing and appropriate. I cant give examples of course as you could track me down and silence me permanently .