According to a well-sourced wikipedia article, those three countries are the only three that have "some adoption", whereas all the others have "partially complete" adoption (UK, Malaysia, Canada, Jamaica), "almost entirely complete" adoption (8 countries) or "complete" adoption (all the rest of the world).
Countries by current metrication status:
- green: Complete
- yellow: Almost Complete
- orange: Partially Complete
- red: Little Adoption
This is also confirmed by one of the wikipedia sources via a little searching.
