Oxford Guide to English Grammar; John Eastwood; Oxford University Press 1994-09
2 Information in a text
a In a text, old information usually comes first in the sentence and new information comes later.
Britain's towns were given a new and an elegant appearance between 1700 and 1830. This period covers the building styles known as Queen Anne, Georgian and Regency, all three of them periods in which houses were very well designed.
Previously, towns had grown naturally and usually had a disorderly, higgledy-piggledy appearance. In the new age, architects planned whole parts of towns, and built beautiful houses in terraces, or in squares with gardens in the middle.
The houses of these periods are well-proportioned and dignified, with carefully spaced windows and handsome front doors. They can be seen in many towns, especially in London, Edinburgh, Bath, Cheltenham and Brighton.
Brighton became famous after 1784 when the Prince of Wales, later King George IV, went there regularly, and later built the Royal Pavilion.
(from R. Bowood Our Land in the Making)
The subject of each sentence is something expected in the context. Usually it relates to something mentioned earlier.
What's the grammatical structure of "all three of them periods" (sentence in bold)?