The etalon boundary-value problem technique for approximately solving three-dimensional diffraction problems has been suggested. On etalon boundary-value problems, we use two-dimensional problems which are capable of exact solutions (for example, by the Wiener-Hopf technique). The field sought along one of the coordinate axes is taken to be locally coincident with that in the corresponding etalon problem. The amplitude coefficient (constant for the etalon problem) is assumed to be varying and is determined by substituting the above-mentioned solution representation into wave equations. Applications of the approach developed for the problem of electromagnetic-wave coastal refraction for a coast with a curved (random) coastal line and for the problem of wave diffraction by a perfectly conducting half-plane with a curvilinear edge have been considered.