WordNet (r) 3.0 (2006):

third class
    n 1: mail consisting of printed matter qualifying for reduced
         postal rates [syn: third-class mail, third class]
    2: inexpensive accommodations on a ship or train [syn: tourist
       class, third class]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "third class":
   Mickey Mouse, base, cheap, cheesy, coarse, common, fourth-class,
   inferior, irregular, low-class, low-grade, low-quality, low-test,
   mean, paltry, poor, punk, second-best, second-class, seedy, shabby,
   tacky, third-rate, tinny