I only heard about what3words last week when it was mentioned in LRO (Land Rover Owner) magazine. So far, it looks interesting.
You can use it via its website and phone apps.
I suspect that TomTom may possibly add support to MyDrive.
iPhone 6s Plus, iOS 12.1.2: iOS CamerAlert v2.0.7
TomTom GO Mobile iOS 1.10.1; TomTom (UK & ROI and Europe) iOS apps v1.29
Garmin Camper 770 LMT-D