We report a multimode analysis of the 3D–2D dimensional crossover for the nonlinear structure, which occurs in the nonlinear regime of the Rayleigh–Taylor instability (RTI). This structure is an array of bubbles and spikes periodic in the plane normal to the direction of gravity. The flow is assumed to be anisotropic in the plane and to have low rectangular symmetry. For regular bubbles, there is a two-parameter family of steady solutions, and we analyze stability of these nonlinear solutions. It is shown that 3D bubbles in RTI conserve a near-circular contour, and cannot be transformed into 2D bubbles continuously. We discuss the mechanism of secondary instabilities of anisotropic RT flow.