[en] A microcalorimeter with event mode capability for time-resolved soft-x-ray spectroscopy, and a high-resolution flat-field EUV spectrometer have been employed at the Livermore EBIT-I electron beam ion trap for observations and wavelength measurements of M1, E2, and M3 decays of long-lived levels in the Ni-like ions Xe²⁶⁺, Cs²⁷⁺, and Ba²⁸⁺. Of particular interest is the lowest excited level, 3d⁹4s ³D₃, which can only decay via a magnetic octupole (M3) transition. For this level in Xe an excitation energy of (590.40 ± 0.03eV) and a level lifetime of (11.5 ± 0.5 ms) have been determined