The real time rain estimates from the TRMM/TMI and the SSM/I Goddard Profiling Alforithm (GPROF), along with the Climate Prediction Center real time IR rain estimates, have been merged to a preparatory version of the TRMM global 3-hourly rain product. This paper discusses the issue of merging NOAA/AMSU rain estimates as an addition into this preparatory product and provides comparisons of rainfall among TMI, SSM/I GPROF and AMSU products. The orbit data of the three microwave sensors were aggregated to 3-hourly gridded rain products with horizontal resolution of 0.25°. The histogram data based on the coincident rainfall among TMI, SSM/I and AMSU estimates from the 3-hourly data series showed about +50% bias of AMSU rain as compared to the SSM/I-TMI rain over tropical ocean. However, over land, the AMSU data showed about -20% bias as compared to the SSM/I-TMI products. The correlation between TMI and AMSU oceanic data is about 0.5. In order to justify the results from microwave rainfall estimates, the TMI rain estimate were compared with ground validation data for a 4-year period. The results indicate a lower (-15%) TMI rain amount over ocean and a higher (+25%) TMI estimate over land as compared to the ground observation.