Due to the increased bandwidth and decreased latency, Optical Network-on-Chip (ONoC) is a good solution for interconnect bottleneck in large-scale Multiprocessor System on Chip (MPSoC). It takes advantage of Wavelength Division Multiplexing (WDM) supporting multiple transactions at same time on different wavelengths. Hence, many senders and receivers can share the same waveguide to increase the total bandwidth utilization. However, multiple overlapped communications on different (close) wavelengths would inevitably introduce non-negligible crosstalk noise on optical signals in large-scale WDM 3D ONoC due to the low wavelength channel spacing and the large Full Width at Half Maximum (FWHM) of the resonance pic of the Microring Resonator (MR). This paper proposes a heuristic algorithm allowing to allocate wavelengths at design time in order to reduce the crosstalk noise. The simulation results show that the proposed heuristic algorithm achieves the same Wavelength Spacing Optimization (WSO) as that of Branch and Bound algorithm for MWD application, 43.75% reduction compared to naive method [10] .