Also called Anglo-Chromatic, these 3-row concertinas have all the
notes of the scale, but still favor playing in, or close to,
the two main keys. The top row has the accidental notes; the
lower two rows are pretty much the same as on a 20 button Anglo,
so it's easy to upgrade from 20 to 30. All of them are single-reed instruments. Compare the different models side by side.
You can read more about
them on "How to Choose A Squeezebox".