1.  trio(n = noun.quantity) 3, deuce-ace, iii, leash, tercet, ternary, ternion, terzetto, three, threesome, tierce, trey, triad, trine, trinity, triplet, troika - the cardinal number that is the sum of one and one and one;
is a kind of digit, figure

2.  trio(n = noun.communication) - a musical composition for three performers;
is a kind of composition, musical composition, opus, piece, piece of music

3.  trio(n = noun.group) - three performers or singers who perform together;
is a kind of musical group, musical organisation, musical organization

4.  trio(n = noun.group) triad, triple, triplet - a set of three similar things considered as a unit;
is a kind of set
has particulars: trilogy, trigon, triplicity, trimurti

5.  trio(n = noun.group) threesome, triad, trinity - three people considered as a unit;
is a kind of assemblage, gathering
has particulars: triumvirate

1.  Three, considered collectively; three in company or acting together; a set of three; three united. [1913 Webster]
"The trio were well accustomed to act together, and were linked to each other by ties of mutual interest." [1913 Webster]

2.  A composition for three parts or three instruments. [1913 Webster]

trio, n. (pl. -os)
1 a set or group of three.
2 Mus. a a composition for three performers. b a group of three performers. c the central, usu. contrastive, section of a minuet, scherzo, or march.
3 (in piquet) three aces, kings, queens, or jacks in one hand.

Etymology:
F & It. f. L tres three, after duo

N  triality, trinity, triunity, three, triad, triplet, trey, trio, ternion, leash, shamrock, tierce, spike-team, trefoil, triangle, trident, triennium, trigon, trinomial, trionym, triplopia, tripod, trireme, triseme, triskele, triskelion, trisula, third power, cube, cube root, three, triform, trinal, trinomial, tertiary, ternary, triune, triarch, triadie, triple, tri-, tris-, tria juncta in uno.

N  music, concert, strain, tune, air, melody, aria, arietta, piece of music, work, number, opus, sonata, rondo, rondeau, pastorale, cavatina, roulade, fantasia, concerto, overture, symphony, variations, cadenza, cadence, fugue, canon, quodlibet, serenade, notturno, dithyramb, opera, operetta, oratorio, composition, movement, stave, passamezzo, toccata, Vorspiel, instrumental music, full score, minstrelsy, tweedledum and tweedledee, band, orchestra, concerted piece, potpourri, capriccio, vocal music, vocalism, chaunt, chant, psalm, psalmody, hymn, song, canticle, canzonet, cantata, bravura, lay, ballad, ditty, carol, pastoral, recitative, recitativo, solfeggio, Lydian measures, slow music, slow movement, adagio, minuet, siren strains, soft music, lullaby, dump, dirge, pibroch, martial music, march, dance music, waltz, solo, duet, duo, trio, quartet, quartett, septett, part song, descant, glee, madrigal, catch, round, chorus, chorale, antiphon, antiphony, accompaniment, second, bass, score, bourdon, drone, morceau, terzetto, composer, musician, musical, instrumental, vocal, choral, lyric, operatic, harmonious, Wagnerian, adagio, largo, larghetto, andante, andantino, alla capella, maestoso, moderato, allegro, allegretto, spiritoso, vivace, veloce, presto, prestissimo, con brio, capriccioso, scherzo, scherzando, legato, staccato, crescendo, diminuendo, rallentando, affettuoso, obbligato, pizzicato, desto, in notes by distance made more sweet, like the faint exquisite music of a dream, music arose with its voluptuous swell, music is the universal language of mankind, music's golden tongue, the speech of angels, will sing the savageness out of a bear, music hath charms to soothe the savage beast.