Due to the continuous integration of power-intensive consumers and producers into low-voltage networks, appropriate equipment models are increasingly important, especially for the calculation of the three-phase network state. Therefore, this paper proposes a generic modelling approach for three-phase two-winding transformers and transformer banks in phase-coordinates. Both asymmetrical and symmetrical transformers of both types can be represented. The different modelling options and the underlying data are comprehensively presented. A large variety of vector groups and neutral point treatments are supported. This paper not only presents the generic modelling approach but also introduces a new methodology for taking into account the asymmetry of three-phase transformers, which is characterised by a particularly low amount of data required. While many transformer models neglect the asymmetrical structure of transformers or require detailed construction data on core and winding geometry, the presented methodology uses only few core geometry ratios. If these data are not available, they can be estimated if necessary or suggested substitutes can be used. For all transformer types, the resulting 6x6, 7x7 or 8x8 matrix can be seamlessly incorporated into the network nodal admittance matrix. The model was validated by comparison with calculation results from established power system calculation programs and measurement results.