The Houston space in Texas is at present served by 3 space codes: 713, 281 and 832. All 3 codes cowl the identical geographic space. A 4th space code will probably be activated on July 1, 2014 as a reduction technique for the aforementioned 3 space code overlay advanced.
See official NANPA announcement that includes implementation dates and different data (PDF doc).
Go to area code 346 web page for a listing of cities served, map and different data.