  We make a detailed time resolved spectroscopy of bright long gamma ray bursts
(GRBs) which show significant GeV emissions (GRB 080916C, GRB 090902B, and GRB
090926A). In addition to the standard Band model, we also use a model
consisting of a blackbody and a power-law to fit the spectra. We find that for
the latter model there are indications for an additional soft component in the
spectra. While previous studies have shown that such models are required for
GRB 090902B, here we find that a composite spectral model consisting of two
black bodies and a power law adequately fit the data of all the three bright
GRBs. We investigate the evolution of the spectral parameters and find several
generic interesting features for all three GRBs, like a) temperatures of the
black bodies are strongly correlated to each other, b) flux in the black body
components are strongly correlated to each other, c) the temperatures of the
black body trace the profile of the individual pulses of the GRBs, and d) the
characteristics of the power law component like the spectral index and the
delayed onset bear a close similarity to the emission characteristics in the
GeV regions. We discuss the implications of these results to the possibility of
identifying the radiation mechanisms during the prompt emission of GRBs.
