Threemonth rhymes with:
billionth, corinth, eleventh, eranthe, millionth, seventh, absinthe, amaranth, billionth, coelacanth, corinth, cynth, drenth, dushyanth, eighteenth, eleventh, eranthe, fifteenth, fourteenth, helminth, hyacinth, hyacinthe, jacinth, jacinthe, labyrinth, millionth, month, nineteenth, ninth, nth, plinth, rhodanthe, seventeenth, seventh, sixteenth, tenth, thirteenth, umpteenth
Threemonth sounds like:
taranto, tarrant, terentia, termed, terminate, terminated, termite, thormond, thormund, thurmond, toriente, torment, tormenta, tormented, tornado, toronado, toronto, torrent, tournament, trained, tramonte, trant, tremont, trend, trended, trendy, trent, treuhand, trimmed, trinidad, trinity, truant, turandot, turned, turnout, tyrant