We simulate the surface electron system over liquid helium subjected to a confinement potential in the electron layer plane. In the solid phase, the two-dimensional system is observed to transform first into a multi-chain system and, finally, into a three chain, a zigzag structure and a single chain. Both longitudinal and transversal plasma oscillations of the three-chain electron system are determined. One of the longitudinal oscillation modes is acoustic, whereas the other two are optical. On turn, all transversal oscillations branches are optical. The theoretical results obtained can be used to identify experimentally the configurations of the low-dimensional electron system over helium under confinement potential.