The single-crystal non-stoichiometric magnetic shape memory alloy Ni(1-x-y)Mn(x)Ga(y) with x = 0.2817, y = 0.2136 is studied using magnetic resonance spectroscopy: ferromagnetic resonance (FMR) and conduction electron spin resonance (CESR). The temperature dependence of the integral intensity, the resonance field and the line-width are measured across the wide temperature interval from 4.2 to 570 K. Three phase transformations are found in this alloy: [Formula: see text] with a Curie temperature of 360 K, austenite-to-martensite (direct with T(ms) = 312 K and reverse with T(as) = 313 K), and a transformation at T = 45 K, suggestive of the spin-glass state. The angular dependence of the FMR signals is measured in the martensitic and austenitic states before and after the martensite-to-austenite transition. The experimental data are used for determination of the magnetization M(m) and anisotropy parameters K(1) and K(2) in the martensitic state. The obtained coefficient K(2) is determined to be not small and, moreover, it is comparable with K(1). The temperature dependence of the resonance signals is also investigated at temperatures significantly higher than T(C), where FMR was transformed to CESR. In the paramagnetic austenitic state (above T(C)) the alloy reveals an extremely intensive signal of CESR, which suggests a high concentration of conduction electrons and correlates with the large value of the magnetic-field-induced strain observed in the alloys of such composition. The temperature dependence of the skin layer depth is found from the sharp decay of the CESR signal with temperature, which is related to the disappearing large magnetic resistance after transformation to the paramagnetic state.