E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Three Estates of the Realm
are the nobility, the clergy, and the commonalty. In the collect for Gunpowder Treason we thank God for preserving (1st) the king, and (2nd) the three estates of the realm; from which it is quite evident that the sovereign is not one of the three estates, as nine persons out of ten suppose. These three estates are represented in the two Houses of Parliament. (See FOURTH ESTATE.)