The ‘Anna Selbdritt’ was a common sculptural theme - in fact there was a huge profusion of this combination of St. Anne with the virgin and child in the 1500s.
Tilman Riemenschneider carved several variations on this subject; see also his sandstone and other wood carvings. There is a much inferior version of this situated close by in the Museum, which you can see here.
This example is delightful for the expressive figures - Jesus and Mary are both incredibly expressive.
