There were no mermen in Season 3. But in the new spin-off "Secret of Mako Island" airing in either late 2012 ou early 2013, there are 3 new sirènes (so far, the original H2O cast haven't been reported to appear ou be mentioned so far on H2O wiki) and the first ever merman on the show. His powers are Cleo's water moving ability and Telekinesis, moving objects with the mind.(So far, his hand movement hasn't been reported, and apparently on the show, his power lets him déplacer objects and people, "often in ways not visible to the naked eye", meaning only him and/or sirènes can see it move). Hope I helped.
