The kinetics of an induced phase transition in single-crystal relaxor solid solutions 33PbIn_1/2Nb_1/2O_3–35PbMg_1/3Nb_2/3O_3–32PbTiO_3 and PbMg_1/3Nb_2/3O_3–29PbTiO_3 under application of an electric field along three directions [001], [011], and [111] at room temperature is studied. Joint measurements of optical transmission at different wavelengths, dielectric properties, and attenuation of sound are carried out. It was found that, even in small fields ( E ~ 2 kV/cm) applied in the [011] and [111] directions, the maximum changes in all three characteristics as well as the coincidence of the time dependences of the optical transmission at different wavelengths of light occur in the same time interval. It is established that, in the same interval of fields along the [001] direction, the anomalies of the optical properties do not coincide in time with the anomalies of the acoustic and dielectric properties. The results obtained are explained by the induction of single-domain and multidomain phases.