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 10.3.3: iOS CamerAlert v2.0.7
Garmin Camper 770 LMT-D - because TomTom GO Mobile iOS is so awful
TomTom (UK & ROI and Europe) iOS apps v1.29; TomTom GO Mobile iOS 1.9.2