Quite surprisingly, even before season 3 has aired, it was announced today that 12 Monkeys will be getting a fourth (and last) season consisting of 10 episodes, according to Entertainment Weekly.
Another surprise is that Syfy has also radically changed the season 3 airing schedule.
EW can also reveal that the third season will debut on May 19 at 8 p.m. ET. […] Syfy will air the entire 10-episode season over three consecutive nights starting Friday, May 19 and concluding Sunday, May 21 — the first four episodes will premiere on Friday, with three episodes airing each night on both Saturday and Sunday from 8 p.m.-11 p.m. ET. [source]
Please read the Entertainment Weekly article for more details.