According to Zootierliste, they are Alouatta seniculus (which I've always known as Venezuelan but Zootierliste calls Colombian) - they are 2 females from Cologne and a male imported from Dallas World Aquarium according to that website's info.
The male also came from Cologne, but was imported by Cologne from Dallas. They are indeed free ranging, together wit saki's, emperor tamarins and goeldi's, they share a large woody area.
Update: Silvery marmosets are a new species to the zoo and will be allowed to range free among the visitors in the same exhibit as the previous mentioned species.