HELSINKI, April 15 (Reuters) - Finland's much-delayed Olkiluoto 3 (OL3) nuclear reactor, Europe's largest, began regular output early on Sunday, its operator said, boosting energy security in a region to which Russia has cut gas and power supplies. Nuclear power remains controversial in Europe, primarily due to safety concerns, and news of OL3's start-up comes as Germany on Saturday switches off its last three remaining reactors, while Sweden, France, Britain and others plan new developments. OL3's operator Teollisuuden Voima (TVO), which is owned by Finnish utility Fortum (FORTUM.HE) and a consortium of energy and industrial companies, has said the unit...