The Collaborative International Dictionary of English v.0.48:

Third \Third\ (th[~e]rd), a. [OE. thirde, AS. [thorn]ridda, fr.
   [thorn]r[imac], [thorn]re['o], three; akin to D. derde third,
   G. dritte, Icel. [thorn]ri[eth]i, Goth. [thorn]ridja, L.
   tertius, Gr. tri`tos, Skr. t[.r]t[imac]ya. See Three, and
   cf. Riding a jurisdiction, Tierce.]
   1. Next after the second; coming after two others; -- the
      ordinal of three; as, the third hour in the day. "The
      third night." --Chaucer.
      [1913 Webster]

   2. Constituting or being one of three equal parts into which
      anything is divided; as, the third part of a day.
      [1913 Webster]

   Third estate.
      (a) In England, the commons, or the commonalty, who are
          represented in Parliament by the House of Commons.
      (b) In France, the tiers ['e]tat. See Tiers ['e]tat.

   Third order (R. C. Ch.), an order attached to a monastic
      order, and comprising men and women devoted to a rule of
      pious living, called the third rule, by a simple vow if
      they remain seculars, and by more solemn vows if they
      become regulars. See Tertiary, n., 1.

   Third person (Gram.), the person spoken of. See Person,
      n., 7.

   Third sound. (Mus.) See Third, n., 3.
      [1913 Webster]

WordNet (r) 3.0 (2006):

third estate
    n 1: the common people [syn: third estate, Commons]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "third estate":
   bourgeoisie, common people, common run, common sort, commonage,
   commonality, commonalty, commoners, commons, laborers,
   linendrapers, lower classes, lower middle class, lower orders,
   lumpen proletariat, middle class, middle orders, ordinary people,
   peasantry, plain folks, plain people, proletariat, rank and file,
   shopkeepers, small tradesmen, the lower cut, the other half,
   the third estate, toilers, toiling class, upper middle class,
   vulgus, working class, working people