3UI lamination cores are used in the manufacturing of three-phase transformers. In transformers with 3 UI lamination cores, each of the three ei lamination core arms bears a coil body. In contrast to the EI sheets, the 3UI sheets cannot be welded since the joints of the E and I sheets are within the coil body. The E and I sheets therefore have to be nested reciprocally during transformer manufacturing. As a result, either one or multiple identical sheets are slid in from one side of the coil body. Nesting machines are available for the nesting of 3UI sheets, which can significantly reduce manufacturing times.