  The branching ratio of the \eta->3\pi decay is an important source of
information on the value of the quark mass ratio 1/R=(m_d-m_u)/(m_s-\hat m).
Furthermore, isospin breaking effects in the decays K->3\pi provide information
on the pion scattering lengths. The cusp effect in the K->3\pi decays is
presently being analyzed by the NA48 and KTeV experiments. From the theoretical
point of view, these processes have been studied by different methods. We
propose a unified and relativistic treatment relying on very general
principles, unitarity, analyticity and crossing symmetry, combined with chiral
counting, in order to construct model-independent representations of the
corresponding amplitudes that are valid at two loops. A general description of
the procedure is given and is illustrated in the case of the \eta decay
amplitude in the leading order in the isospin breaking.
