The Collaborative International Dictionary of English v.0.48:

Three \Three\, n.
   1. The number greater by a unit than two; three units or
      objects.
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   2. A symbol representing three units, as 3 or iii.
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   Rule of three. (Arith.) See under Rule, n.
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The Collaborative International Dictionary of English v.0.48:

Three \Three\ (thr[=e]), a. [OE. [thorn]re, [thorn]reo,
   [thorn]ri, AS. [thorn]r[imac], masc., [thorn]re['o], fem. and
   neut.; akin to OFries. thre, OS. thria, threa, D. drie, G.
   drei, OHG. dr[imac], Icel. [thorn]r[imac]r, Dan. & Sw. tre,
   Goth. [thorn]reis, Lith. trys, Ir., Gael. & W. tri, Russ.
   tri, L. tres, Gr. trei^s, Skr. tri. [root]301. Cf. 3d
   Drilling, Tern, a., Third, Thirteen, Thirty,
   Tierce, Trey, Tri-, Triad, Trinity, Tripod.]
   One more than two; two and one. "I offer thee three things."
   --2 Sam. xxiv. 12.
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         Three solemn aisles approach the shrine. --Keble.
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   Note: Three is often joined with other words, forming
         compounds signifying divided into, composed of, or
         containing, three parts, portions, organs, or the like;
         as, three-branched, three-capsuled, three-celled,
         three-cleft, three-edged, three-foot, three-footed,
         three-forked, three-grained, three-headed,
         three-legged, three-mouthed, three-nooked,
         three-petaled, three-pronged, three-ribbed,
         three-seeded, three-stringed, three-toed, and the like.
         [1913 Webster]

WordNet (r) 3.0 (2006):

three
    adj 1: being one more than two [syn: three, 3, iii]
    n 1: the cardinal number that is the sum of one and one and one
         [syn: three, 3, III, trio, threesome, tierce,
         leash, troika, triad, trine, trinity, ternary,
         ternion, triplet, tercet, terzetto, trey, deuce-
         ace]
    2: one of four playing cards in a deck having three pips [syn:
       trey, three]