Regulation (EU)548/2014 does not explicitly exclude transformers with three or more windings. The definition of transformer refers to 'a static piece of apparatus with two or more windings which, by electromagnetic induction, transforms a system of alternating voltage and current into another system of alternating voltage and current usually of different values and at the same frequency for the purpose of transmitting electrical power'. It can therefore not be concluded that three-winding transformers are outside the scope of the regulation. Tables 1.1 and 1.2 in Annex 1 are therefore applicable to this type of transformers.
However, existing harmonised standards do not describe explicitly how to measure load and no-load losses in three-winding transformers. While these standards (adopted in 09/2015) incorporate specific testing provisions for three-winding transformers, testing methods should follow, as closely as possible, existing standards.
Disclaimer: Please note that the European Commission cannot provide a legally binding interpretation of the EU legislation, as this is the sole competence of the European Court of Justice. Any remarks from the European Commission services are without prejudice to the position the Commission might take should related cases arise in a procedure before the Court of Justice.